Because the world is a pretty shitty place. The stress that actors and people in the spotlight are under has to be even worse....
I think it's more that uber-creative types are by nature more prone to deviance (in the technical sense of the word). They're more likely to think, feel, and do things that ordinary folk aren't.
Add to that the idea that they're surrounded by yes men, and know they "should" feel on top of the world, but feel no real sense of purpose, accountability, etc, and you have a recipe for disaster.
