Why do girls starve themselves for vanity?

to the point of obsession, for the deluded idea they might look fat, and this isnt the odd 1 or 2 girls its most of them, its unhealthy and quite frankly weird and off putting, are girls that obsessed with the way society sees them? i guess they must be, what happened to self esteem, pride,knowing what guys really like curves and the love of a hot meal for crying out loud