He's done nothing to show that he deserves to start.
I just don't think he's as talented as people think he is. He has an inconsistent jumpshot, a rudimentary postgame, a poor first step, and an inability to change directions with the ball. He's also not rebounding, but we expected this already.
If he's ever going to be reliable as a scorer, then he desperately needs to improve his footwork. In my opinion, that's the root of many of his problems. Better footwork would give him a greater ability to produce off the bounce, a much-improved ability to score in the post, and would improve his jumpshot by helping him with balance and mechanical consistency.
Until then, he's going to be a guy who struggles to create offense despite excellent running and jumping ability for his size.