Ratings inflation is one cause.
However, todays players are drawing on quite a bit of theory that's been developed since the 60s, including many contributions made by engine analysis that wasn't available back then.
Assuming each of the sets of players were utilizing the resources available to them in their respective eras, there would be little contest. Today's players would dominate.
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The top 5 are all currently over 2770 yet there were world champions in the 60's that are rated much lower, in fact they wouldn't even get into the top 50 these days. Even though the rating system is not perfect, do you really think that there are at least 50 players or so that could beat the likes of Spassky and Petrosian, who were world champions in their own right. The 60's had a lot of great characters that were capable of producing great chess ideas over the board (leonid Stein for example), whereas today's games feature players (even Anand) who follow theory well, then immediately go off the rails and lose the second they forget the theory. Today's game is too theory-laden, and today's players lack personality. We currently have very few players that will be remembered in history unlike the 60s. What do you think? Are today's players that much better than those in the 60's?