I think (hope!) it's probably a combination of factors - firstly that the top three really are exceptional, probably the best men's tennis has ever seen; vast improvements in modern sports science - nutrition, strength training and conditioning, etc; being much more selective with their schedules (especially Federer of late); improvements in equipment; and some weird quirk that has meant the younger generation hasn't really come through and the older ones have been able to stay at the top with a periodic cycle of displacing each other as they get injured, struggle with form, and so on.

Or maybe it's just drugs, though they get tested regularly and punished very severely (but you could say the same about cycling...).