What don't people get about young players
They learn to play and prove themselves to their coaches in practice, the weight room, shoot arounds, walk throughs, video rooms and testing to EARN playing time
They don't go to the D-League, play well and GET NBA time to learn the NBA game
They have to prove they know the plays, the defensive rotations, how to fight through screens and picks, how to double team and fall off into a proper rotations, proper offensive spacing, where to find the open man, how to run the break properly, whether he can rebound and box out properly, how to make the outlet pass, etc. etc. etc.
If they aren't doing or know how to do those things behind the scenes, what makes you think they will learn to do them at an NBA speed against professionals that will make them look silly
Walker failed in those three minutes not because he doesn't get more time but because he doesn't get it in practice