Do you not get how important getting more knowledgeable is? Watch a college game and notice all the mistakes players make, the college game is much sloppier than an NBA game because the players have lower IQs. Another way players improve is by getting stronger and faster once they get to the NBA. Having personal trainers and dieticians attatched to NBA teams allows players to go further then they ever got in college.
Using WIS ratings as an example, imagine 2 players that have all the same ratings in skill areas, however one player has 70ath/70sp and a C- IQ, the other has 90ath/90sp and A+ IQ, which player is going to be better? That's the type of improvements 21+ year olds make, not necesarrily with skills.
The fact that I used to cover high school basketball in NYC, I have seen high school kids get no better after the age of 15, I've even seen 16 year olds regress. There's an easy example from college basketball of a kid regressing, Julius Hodge won ACC POY as a junior, shot slightly over 50% his junior year shot a respectable 36% from 3, and for his career was over 80% from the line. His senior year he shot under 44% from the field, under 25% from the field and shot 60% from the line. An example of a player who just never got better even out of high school is Kelvin Torbert at Michigan State. He came in as a top 3 recruit stayed 4 years and never even made an All-Big Ten squad.
What if players started getting worse at some point in their career, would people call that unrealistic? The fact is, it happens, and players often stop improving, and it's actually very common for seniors to only be marginally better than they were their junior seasons, or no better at all.