I think that the media and peers play a large part in it. Maybe low self esteem or a need to control an aspect of their lives (this reason wouldnt be the majority of the girls). From an evolutionary perspective it doesnt make any sense, for the reasons you stated above. When you're too thin you look unhealthy, and no one wants to invest resources in someone who couldnt raise healthy offspring. I'm not one of those really skinny girls though so I can't give you a solid answer on anything.