It's a good question, OP. I'm older now; was pretty, slightly overweight most of my life and hideously fat now. I don't even want to THINK of what people think when they see me; I AM trying hard to lose weight for health reasons.
I was always shy; my mother and aunt would tell me I was pretty and I didn't completely believe them; took to age 34 and friends complimenting me (who hadn't seen me in years) for it to sink in. Again, "somewhat pretty" - NOT beautiful. After 37, it all started to fade, hee-hee!
ANYway: again, you pose a good question. In nature, I have always read it's the MALE of the species who is more stunning-looking: lions, etc.; "peacocks".
I wonder if some of nature carriers over with humans: any way you slice it, woman are overall, "weaker"(I know everyone will hate that word; I'm sure I could find a better one, sorry) - smaller, less body mass, less physical strength.
SO in nature the "weaker" animal seeks nurture from the strong? Women need men for protection (in the past; the feminist "women need men like a fish needs a bicycle", hee-hee! is more apropos in many ways), perhaps? THere are, it seems, many more women than men? So women instinctively want to "look their best" to capture a man?
(the "??" are 'cause I don't really know.)
I have read - sorry, this will sound negative - magazine articles that "pretty people" in ge3neral, have more success in life. There have been experiments where babies are given photo albums of "good-looking" people, and the babies, laugh, smile, coo, and point more to the "pretty" pix. Again, this isn't MY opini, but I have read about it.
Why are there beauty pageants? Why do women get all "gussied up" for proms, going out; hell, their WEDDINGS?
Am getting off track, sorry. Perhaps men (straight ones) by and large feel - quite likely subconciously - that women are not their physical equal; they can stand the thought of women being their INTELLECTUAL or wage-matching equal, so:
A woman might just as well have SOME damn thing going for here, ergo: to look good.
Is better these days then in the past, I would say, based on watching "Mad Men" and the recent "Vegas".
Sorry to go on and on but this is the best question I've seen on DL in a while!