In college, a player gets a lot more practice time. The practice to game ratio is higher. In the nba, a team can go a week or so without practice.
NBA success comes from talent, hard work, size and confidence. In college, a player can feel more encouraged about learning post moves, off hand stuff, other fundamentals, when playing against lesser competition. In the pro's you rarley see much significant low post development.
But i agree that if you are projected to be picked in the lottery, you should probably declare for the draft. The lure of financial security is too high. At that point you just hope the raw player is playing with good teammates and a good coach in a well run organization.