Probably because obesity can be caused by other things such as bad metabolism, malnutrition, and poor sleep. There is such a thing as Binge Eating Disorder which [i]causes[/i] obesity, so that's analogous to what you're talking about.
Obesity is a state defined by a particular weight. Anorexia doesn't necessarily mean thin, it's a disorder about one's relationship to food, the body, etc.
