College is great for players who have the dedication and foresight to develop in areas they are weak. Steph Curry came back each year in college focused on developing more aspects in his game to make him more than a shooter and valued more by the NBA. He came back his junior year committed to developing as a point guard not wanting the NBA to evaluate him as just a shooter. If a player is focused enough , determined to diversify their game, turn weaknesses into strengths then coming back is a good thing because it maximizes their NBA value. The more the NBA is willing to invest $ wise in your first contract , the better chance you can stick around to prove that worth. If a player lacks the motivation, believes that college becomes more of a distraction from their goals than an opportunity to improve, has no room for improvement believing that leaving maximizes their value over staying or needs money now regardless if fully ready - then leaving makes sense.