Because she DOESN'T WANT TO. Not everybody wants to be a stick thank you very much. If she wanted to be skinny, she would be. It's not like she couldn't afford a great traitor or surgery.
"I want it to be honest and genuine in expanding my wings and doing something really, really different. A crackhead on the street or a drug addict. Or something crazy and meaty and juicy. Something that really took me out of myself."