I don’t think they “improve” as much as they used to.

I don’t have any data on this of course. But you did say it was a “philosophical” question. So that opens the door for philosophizing.

I think there could be many reasons for this. Not enough practice time which means not enough reps. More is known about the players when they come out, so you know what you’re getting before they walk on the field thus they can’t “improve” in our eyes because we already knew what they could do. More specialized positions. Players do the job that they are assigned. Nothing more, nothing less. Players are training so much for so many years to make it to the league that they are already at their ceiling when they get there. Big business. More pressure to perform now means less time to wait for improvement.

Just ideas.