Eating Disorders pt 1

I do not understand how someone has an eating disorder.  I do not understand how someone who is diagnosed with anorexia or bulimia can’t just start eating more or quit making themselves throw up?  I have always viewed eating disorders as more of a personal choice than really a disorder.  I guess my question is why can’t someone who is diagnosed with an eating disorder control how often they eat, how much they eat, and what they eat?  This has always been a great mystery to me.  I hope someone can help clear this up for me.  Thanks!