Because any player with any semblance of talent gets the fukk outta college the first chance they can.
The only answer.
95%+ of these dudes will never do anything in the league or even get drafted.
The gap between "very good college player" that can whoop EVERYONE in LA Fitness runs....versus Luka Doncic is like the gap between Donald Trump and Tyreek Hill running the 40.
College guys can't really generate offense and get past defenders. They just swing the ball from side to side and then take a contested 3. Then the whole team crashes the boards and tries to get the put back. If breh gets hot from 3, they'll just keep feeding him.
A good example of talent gap is Kevin Durant at Texas. His one year in college....it was clear he was soooooo far beyond anyone else it wasn't even funny. Imagine LeBron at Ohio State, Texas, Duke, Florida etc., in 2003. He would have looked like 1980s Magic Johnson playing against middle school brehs.
Luka in college? Omg....could you imagine a player like him with his shyt talking and game. 6'7" chunky but strong like an O-lineman? Dude would have averaged 40/15/15.