Because our cultural mirror is cruel:

"Maybe this all seems funny, or trivial, but it's really not. It's about what girls want to be, what they're told they should be, and how they feel about who they are. . . I don't want them to be empty-headed, self-obsessed, emaciated clones; I'd rather they be independent, interesting, idealistic, kind, opinionated, original, funny -- a thousand things before 'thin.'

I'd rather they didn't give a gust of stinking chihuahua flatulence whether the woman standing next to them has fleshier knees than they do.

Let my girls be Hermiones, rather than Pansy Parkinsons.
Let them never be Stupid Girls. Rant over."