Because being fat isn't "cool," despite what the HAES people may say. Phil only really cares about image. Yeah I think he does hate himself and what he grew up with. I'd be pretty pissed off too if I had to live his life. Most people would. But rather than try and change it, Phil just throws a temper tantrum like a spoiled child. He's really too dumb to improve his life. Look at all of his lies; he's trying to present an image of what he thinks is cool and edgy, and being fat isn't a part of that.
Alcohol is probably too adult for him, which to be honest, is a good thing because if he was an alcoholic he'd piss his money away even faster than he does now, and he'd probably die a lot sooner. Thankfully Phil isn't into alcohol or drugs. He really does have the tastes of a 12 year old boy; he wants to eat pizza and drink lots of soda, and probably watch anime while doing it. Beer would be too "icky" for his tastes.