Why don't we have thinkers in the present time (or recent times) that have the same caliber as Albert Einstein, Archimedes, Socrates, Shakespeare, Freud, Aristotle, Plato, Darwin, Popper and all the other great minds over the past few thousand years?
I have heard two explanations for why present philosophers don't appear to be as important as those of the past:
Because we have much more evidence of the impact of old ideas than we do of the impact (or potential impact) of new ideas. Therefore it's much harder to know the relevance of new ideas and much harder to find any consensus of what will turn out to be important in the future. It is easy to deny an idea's relevance based on the lack of evidence and so many will deny it until it is impossible to do so. Maybe for good reasons, but maybe because of my second explanation for why present philosophers don't appear to be as important as those of the past.
People seem much more comfortable giving away credit to dead people than people who are alive. (I don't want to go into the potential psychological reasons for this but there could be plenty of alternative explanations for this phenomenon.)
In older days, the amount of verified and certain information, from the standpoint of Science, was less. This may have allowed individuals to ignore more of the facts that society discussed, and thus arrive upon general principles more readily.
In recent times, facts have become very granular and multitudinous. Asserting general principles is thus far more difficult because of the amount of learning required.
Alternatively, you can think of the low probably of someone selectively ignoring, correctly, much of the spectrum of facts, and thus having a 'manageable' field of view upon which to derive general principles.
I think we, as a species, as a group, are better at thinking now than ever before, due to technologically accelerated communications and info processing, and simply standing on the shoulders of giants (AKA the accumulative advantage mentioned in other answers here).
The result of this better thinking is more steady, evolutionary progress at the expense of revolutionary discoveries (like relativity, evolution, etc.). (Don't revolutions point to a failure of the old guard?) Without revolutions it's more difficult to standout. There's no decline in great thinkers (quite the opposite I say), just a decline in exceptionally newsworthy events in the thinking sector.
(What's been in science news lately? The Higgs Boson. It will be interesting to see if Higgs joins the pantheon at some point.)