Since the rise of patriarchy, about 5 thousand years ago, beauty became a primary asset for women (because she was stripped of all other ways to define her worth). Girls have it drilled into their heads from a young age that the most important thing they need to be is pretty - not only to attract boys, but to succeed in every area of life. But that's also a comparatively recent development - from about the past 4% of human history, so not a timeless human endeavor.