Here's the thing: Models aren't met to look like "normal" girls. They aren't supposed to be representations of normal people. If they were, they would just grab regular random girls off the street. The truth is clothing, especially haute couture or expensive designer clothing, looks best on tall, skinny girls. Much like a mannequin. The fashion industry is all about portraying a fantasy of sort, that's rare and unique. Because that's what expensive luxury is all about.
I hate the notion that skinny or underweight = anorexic or unhealthy. HELLO, some people out there are NATURALLY thin, and many of the models on the runway ARE naturally thin - which is how they got discovered in the first place: for being, tall, thin, and looking like a model. I had how people claim just because your skinny, your not healthy. It's ridiculous. Not everyone's bodies are the same.
It'd be reasonable to use a regular or plus sized model for an average, mass commercial brand like Target or Macy's. But when it comes to designers, that's not the look they want. Because people who buy designer clothes don't want to look like everyone else.
There has been more body diversity in the modeling industry lately though, which more curvy-skinny girls on the runway, not just stick thin ones.
Take Lara Stone, for example. She is the #1 model in the world right now. And she's more curvy than most other runway models.
And also Barbara Palvin and Crystal Renn

Compared to other runway models...