I think the most collected baseball player is Mickey Mantle. Along with the popularity of cards from his playing days, there has been a steady stream of modern issues from various card producers since shortly after his passing.
Specifically, Topps has included him in a plethora of their sets since 1996. As such, an entire generation of collectors who never actually saw him play have pulled Mantle cards from packs. And these issues tend to be valued a bit higher than their counterparts, thus drawing more attention.
For example, This issue from 2011.
(Not mine anymore...although I was lucky enough to find it in a pack of base brand Topps)