The 3 year timeline for wrs, which is a bit outdated nowadays, is based on route-running skills coming out of the college game. It's not a magic formula.
The physical abilities are the physical abilities for the most part. Beyond getting stronger, the NFL represents a big jump in technique to win.
It's the same situation faced by most players, in that where can they develop and grow their technique, because they are no longer just going to be able to count on being the best athlete, or the most physical, and dominate a lesser foe based only on physical attributes.