Models are freaks of nature. Only a freak of nature can simultaneously be 5'11" tall and weigh 115 pounds. I think it's conspiracy, because fabric's expensive but tall people look better on TV than short people, so designers probably prefer tall anorexics because they can use less fabric and the tall people can still look better on TV.
A friend of mine once explained it best:
"Models are simply human coathangers, the clothing is simply designed to drape off of them so that the clothing is the centerpiece, and the shape remains neutral."
If you think about it in this context, it makes sense. However, yes, it is still fucked up. And I think that most models aren't very attractive people.