It's simple economics really, supply and demand wrt Rooney's wages.
I would label last year as a good season for Rooney, all things considered and in comparison with the rest of the players. Rooney was our best outfield player last year, no question, nobody else came close except a breakthrough 18-year old. But that alone doesn't justify his 300k p/w of course. He's our most marketable player by a long shot, he may not be liked by football fans, but sponsors line up for him.
I would argue that he has been consistent. Over the last 5 seasons he's averaged something like 18 goals, the season before last when we accommodated RVP and he was played largely in midfield he still scored like 13 and made 15 assists, and thats still with injuries. He's one of our top scorers for a reason. He may not play well every single game, but honestly none of our players do.