Not even it's as simple as "he doesn't try". I don't think you can watch him play and think that he's not putting enough effort in.
I think world class players need to be surrounded by great players and a great manager, and given the space and responsibility to thrive. Rooney has none of that at United.
I think if he were at a Bayern, Madrid or a Barca we'd feel very differently.
It's funny, Rooney always struck me as someone motivated purely by football (by achievement, not necessarily loyalty), not money. I think his upbringing, or maybe the people around him, have led him to compromise his football for money - and I don't think he's happy with that.