He never had to play in an actual offensive system like Billy's. He was way bigger and way more athletic than the small school kids he played against, and pretty much did whatever he wanted. He's honestly not the sharpest tool in the shed, either, and the learning curve seems to be enormous for him. He has tremendous raw talent and upside, but he's struggling to learn how to play the game in a team concept.
I really wish Billy wouldn't try to play Walker with his back to the basket so much, but that's his system. If Walker could get better at running the pick and roll, he'd get some wide open dunks and layups (which he does on occasion), but he'll never be a traditional center like Patric Young was. I wish Billy would let him work more as a stretch 4 where he could put the ball on the floor from a little further out and work towards the basket, but he obviously doesn't want Walker in that role.