Because they're raised that way.
No, really. How many of you have seen a girl's life from birth? From the moment they can speak and understand, they are taught to look after their appearance and that it is the single most important thing about their life.
In their younger school years, they are viciously ridiculed by other girls and often by boys for having an unusual or less-than-pristine appearance. It's not uncommon for kindergarteners to forget to brush their hair on the back of their head(failing to see it in the mirror) and end up having a horrible day as people mock them.
Going into high school, the rat race for friends, cliques, and social status begin. How does a girl get ahead in this? Certainly not good grades, being a benign housewife type, but by being pretty. They learn sex as a way for men to appreciate their appearance, and so become whores, believing in their naive minds that getting a lot of sex means they must be pretty, soothing their inflamed self-consciousness.
As they get into their twenties, the rising responsibilities of young adults make having children a very poor idea. Why would you have children when you don't even have steady employment? Maternity leave is going the way of the dinosaur, and it is no longer reasonable to expect a single man of the household to be able to support her for nine months. They need time to get college degrees, save up money for having a child.
Of course, having a child ruins your beauty as you face hormonal changes and stretch marks, so women face strong incentives to not bear children. When they do, it inevitably causes them a great deal of self-consciousness and despair as they see the one thing they built their life around disintegrate.
Of course, they now have this beautiful, young baby girl with a chance to start over and recreate her beauty vicariously. Why, she even looks just like you. We should teach her early the importance of makeup and good hair…
Your answer is; How could they be anything else in the current culture?