The hair on our head is the one clear-cut case of ornamentation like the peacock's tail. It just keeps growing and growing and growing for no reason, other than to get noticed. In infancy, it's short, wispy, and subdued in pigment. And in old age, it's short, wispy, and subdued in pigment. It's only during adolescence and early adulthood that it's thick, long, and richly colored, another sign of hair's role in dating and mating.
It's no surprise then that in cultures where women's sexuality is tightly controlled, they minimize the appearance of their hair. This includes not only cultures where a woman has her look policed by others (typically by older women, not by men, who have more important things to do), but also those where the woman just doesn't feel like displaying her sexual maturity in the first place.
Other parts of her appearance, both natural and artificial, may serve other functions and are not necessarily so rigidly regulated. Colder weather tends to favor more skin being covered up, and hotter weather less. Frequent walking throughout the day selects for shorter hemlines on flowing robe-like clothing, so that strides may be taken less clumsily. But since all that hair seems to serve no other function, it can be pinned up and covered up without affecting the woman's daily activities.
We don't live in a pastoralist culture of honor where women's modesty in appearance is enforced mainly for married women, and where unmarried women are allowed to let their hair down so they can attract a husband before it's too late. Rather, we seem to be moving more toward the agrarian extreme where women don't mind dulling themselves down because they aren't chosen on the basis of looks so much as their ability to produce wealth, whether by toiling out in the fields or by getting a degree and a career. We're becoming less Mediterranean and more East Asian.
And it's not just that girls aren't putting much thought into looking pretty, but are otherwise well adjusted psychologically about their sexuality. They come off as awkward. Just read the faces you see in a google image search for "slouchy hats". Their eyes are lost in that whole hipster fairy-child dream they have, of returning to a pre-pubescent stage and roaming the empty fields alone, or at most with a drab-looking non-boyfriend who won't ever make a move to touch her "down there".
See the lookbook of any store aimed at older teenagers, like Urban Outfitters, American Eagle, or Hollister. The pitch is, "The perfect clothes for indulging your awkwardness and lonesomeness." What's truly bizarre is that they're not targeted at the ugly and socially invisible girls. Everyone in the Millennial generation is pretty dorky, so even the cute girls in the dorm fantasize about the freedom to feel awkward with no one else around to notice them and make them feel, uh, awkward.