Because generally men have short hair and women have long hair. Long hair is seen as feminine. The universal standard of beauty tends to be light/tanned skin, big ass, big breasts, full lips, long hair etc.. These are seen as womanly traits.
Each race is naturally blessed with some of these traits that's why black women generally want lighter skin (not white), long hair, smaller nose etc.. And white women want tanned skin, fuller lips and bigger ass. They want these things because their race is not naturally blessed with them and all women are trying to achieve this universal beauty. Mind you most women are happy in themselves, and some are lucky to have traits their race is not naturally gifted with but those women don't garner as much attention as the insecurer ones.