Gregory Lewis has a very odd guest post up at "The Daily Nous". But, on the other side, it does a good job of pulling together a number of threads I've seen separately elsewhere, so it is handy for saying something all at once.
(1) The basic question asked is why the 'greatest philosophers' live so far in the past. But there's no actual standard by which greatness is being measured here. The measure seems to be mostly what contemporary philosophers would tend to apply that title to; that is, it is an indirect extrinsic measure by way of reputation. We aren't directly talking about greatness at all. This raises several problems.
(1a) If we're measuring reputation, why would one expect there to be very many contemporary philosophers with reputations on the level of Plato, who has been around almost nonstop for well more than two millenia? How would most philosophers even build up a reputation like that over a short time? Even Plato in his day did not (we might say) have the philosophical reputation of Plato; he was just one of many students of Socrates. Plato's reputation is partly due to the fact that he has kept being valuable in one way or another almost nonstop since his own day.
(1b) If you ask academic philosophers about reputations, what you will get will reflect philosophical pedagogy. All the people who get on the list will be philosophers who are useful for contemporary philosophical teaching. This will be due to a combination of factors in various proportions depending on the case -- teachability, the usefulness of a philosopher as a gateway to different philosophical field, the influence of the philosopher on those fields, the usefulness of the philosopher as a reference point for navigating contemporary philosophical research and discussion, are just three factors that obviously strongly favor Plato over almost anyone else, for instance. Frege pretty clearly gets on the informal list Lewis mentions primarily because analytic philosophers regularly use tools that can be traced to him. And so forth.
(1c) If we're just talking about a list of 'greatest philosophers', without any further specification, we're asking something very generic and general. It will favor brilliant generalists over brilliant specialists, influential people over people who have not been as influential, people we happen to know about over people we don't, people who have lots of extant works over people who don't. Obviously none of these are requirements, but if you ask, "Who are the greatest philosophers?", these biases are built into the question to begin with. Thus these lists give us people like Plato (who covers an immense array of fields) and Aristotle (who even in the ancient world was jokingly said to know everything), major generalists who have been almost continually influential on parts of the discussion and keep being explicitly discussed. You can be extremely influential (like Zhu Xi) and yet virtually unknown to the people making such lists; you can be extremely brilliant and yet too specialized to come to mind if the description is just 'greatest philosophers'; you can be extremely brilliant but out of fashion at the time that they make the list; and so forth. Lewis says that the list of greatest philosophers is partly explained by "polymath premium" and "forefather effect". Well, obviously these are found here and obviously they explain nothing; original generalists of great influence is literally just what we usually assume we are talking about when we talk about 'greatest philosophers' with no further specification. Others might get in if they are especially important to us, but that's about the only other thing we might take the question as asking.
(1d) By definition, great philosophy is something that endures. About whom do we have better information about the ability of their work to endure, Plato or someone in the philosophy department at the nearest college? Lewis at one point says that the list of greatest philosophers is partly explained by 'retroactive esteem'. This is not an explanation of the list of the greatest philosophers at all; the list of greatest philosophers is nothing other than a list of philosophers who are the objects of great retroactive esteem, for some reason or other. Even if we added someone currently living to the list, we could be dealing with nothing other than retroactive esteem for what they had accomplished.
(2) Lewis talks about low-hanging fruit; and this is, I think, a case of a metaphor running away with its argument. When we call something 'low-hanging fruit' we can mean that it is easy, or that it is unimportant, or that it is early (i.e., one of the next thing in some kind of order). A major problem is that in intellectual matters, these three easily come apart. Whether or not an intellectual question is difficult or easy to answer does not tell us anything at all about how important the answer is. The earliest questions, in the sense of being the first that you'd have to ask, are not always the easiest questions. And sometimes the most important things are also the obvious places to begin. And the danger here is assuming that we can simply conflate the three concepts. Lewis, for instance, explicitly conflates earlier with easier. This is an obvious error.
Plato and Aristotle are obviously going to take some of the earlier steps; that logically follows from the fact that they are earlier and took some steps. We learn nothing from this. The earlier steps are not necessarily easy; they could, for all the bare fact of earliness tells us, be the most difficult ones. The earlier steps are not necessarily of lesser importance; they could, for all the bare fact of earliness tells us, be the only important ones.
It's likewise important to keep in mind that 'low-hanging fruit' in intellectual matters is entirely relative, and not sharply defined. New discoveries mean new low-hanging fruit. New problems can mean new low-hanging fruit. And, what is more, in intellectual matters, fruit may be low-hanging from one direction and not from another. Relative idiots in mathematics can be taught to solve mathematical problems that four thousand years ago would have taken the greatest mathematical geniuses in the world to answer. How? As Descartes saw, through the refinement of method. But the fact that any logician today can handle syllogisms does not tell us much about their logical acumen in comparison with Aristotle, despite the fact that Aristotle probably did have a harder time with syllogisms. But that's because Aristotle had to figure out what syllogisms were, how they worked, and the methods for tracing out how they worked, almost from scratch; there could hardly be a logician alive today who actually did that rather than being taught Aristotle's answers (or refinements of Aristotle's answers). Inventing syllogistic is a sign of genius; learning it, not so much. Basic syllogistic is perhaps low-hanging fruit today, after Aristotle; before Aristotle it wasn't even obvious that it was on the tree.
(3) But there is a more important issue involved here. Lewis suggests that there is an incongruity between the fact that the 'greatest philosophers' tend so often to be long-ago and the fact that the population of the earth is so much greater. But there is nothing, literally nothing, in the history of philosophy to indicate that philosophical work correlates with population. Indeed, we know that it can be drastically affected by infrastructure. It is not really surprising that there are more significant philosophers in the burgeoning Spanish Empire in the sixteenth century than in the late eighteenth. Sixteenth-century Spain is highly educated, has a lot of resources, has a cultural emphasis on the general social importance of philosophy, has networks in place encouraging the interaction of philosophers in Spain and its colonies with philosophers outside it, is dealing with new problems in fields like law that desperately need solutions and that philosophers are actively encouraged to discuss by society at large. Late eighteenth-century Spain, despite some talented philosophers and universities still chugging along, does not actually have most of these advantages.
Social context also seems to have an effect. Students in Plato's Academy or in the Stoa did philosophy pretty much all the waking day in a population that already did a lot of philosophy and for completely independent reasons actively encouraged arguing about practically everything. How much time does the philosopher of our day spend grading the same things over and over, or doing paperwork, or watching TV, or the like? How much does our society actively encourage philosophical discussion? How easy is it for someone interested in philosophy of X to find other people interested in it and to spend days and months discussing it with them, at relative leisure?
In early modern Vienna, you could find some of the greatest composers in Europe just by going down to the beer hall. It's not an accident that it was Vienna; Vienna was music-mad. This doesn't mean that most of the music in Vienna was great music; most of the music in Vienna was probably pretty kitschy. It did mean that great musicians went to Vienna because the Viennese appreciated music. It did mean that talented musicians in Vienna could easily find other talented musicians to learn from. It did mean that a lot of new approaches to music were being tried out in Vienna. It did mean that it was possible in Vienna for a musician to spend lots of time on music and nothing but music. And we can see this in other ways. Likewise, it's not a surprise that so much of Western philosophy goes back to Athens, in particular. Athens had the encouraging culture, it had the supporting institutions, it had the pressing need for solutions to problems (particularly in politics), it had a general belief that philosophy was some kind of important or other, it had the resources directed in the right way. That we find so much philosophical work done in Athens and so relatively little done in Carthage has absolutely nothing to do with their respective populations.
One of the assumptions the argument has to make is that great philosophy is cheap and easy, so that it's always equally possible. But everything we know about the history of philosophy suggests that the conditions for having a lot of major enduring philosophical work done at a given time and place are fairly fragile and difficult to build and maintain. Yes, modern America is massively larger than Classical Athens; but there's no reason to think that it's anywhere remotely as efficient at encouraging a lot of philosophical work that is likely to be valuable for centuries.
(4) If one looks over it all, there is an obvious problem with the whole discussion of whether Plato is really great; not once does it look at or assess anything Plato actually did. Occasionally, moving out and about, you'll find an idiot who blusters about how Plato is an idiot; I have never met any such person whose reading of Plato was even competent. I once came across someone arguing that Plato was famous because he had been a big fish in the small pond of Classical Athens. The obvious nonsense of this is seen once you actually look at the facts of the history of philosophy; Plato has been a big fish in every body of water. These sorts of things are always speculations divorced from actual evidence.
It reminds me of the people who were saying a while back that we live in a Golden Age of philosophy. Historically, the kinds of periods we call intellectual 'golden ages' tend to be periods of flourishing and innovative educational systems in which intellectual work is highly prestigious and generously rewarded with material benefits, and in which the intellectuals work both cooperatively and competitively on problems of widespread social significance; but the kinds of people who say that we are in a Golden Age don't ever look at any of that. They rarely dirty their hands with any evidence at all, and when they do, it's virtually never based on the bits and pieces that we have historical reason to think are relevant to 'golden ages'.
But from the perspective of the history of philosophy, it's the actual evidence alone that matters. To historians of philosophy belongs the realm of the dead; sooner or later everyone comes to us. When Minos judges you, O denizen of the 'Golden Age', it will not be in the finery you deck yourself. What matters is the evidence, and your bluster makes not one whit of difference in the scale. And when you are remembered, O philosophical Ozymandias, it will not be for your greatness in your own eyes. All that will matter will be the evidence in the sand.