Because players get paid NOT to, that's why. Not only that, but who's to say this is a new development. Rooney has no reason to smoke at any time. It's been noticed that his fitness at the end of the season wasn't great, nor was his finishing or even his performances.
What's he done over the summer to correct that?
1 team **** up after Champions League, a **** up with his family and friends on a boat for a week and a **** up at the wedding and stag.
Since then he's been in Vegas drinking and smoking.
For a player who was short on fitness, belief and quite honestly, ability, I'd have expected him to be really working on it over the summer, he doesn't have the benefit of the Euro's to help like the others, so he should be working hard himself, instead we've seen him doing everything but.