It's something of a puzzle why beautiful women use all sorts of make-up, sink big bucks into their hair style, collect expensive jeans, and so on. If a 9 could leave the house without even showering and turn every man's head, why does she spend so much time, money, and effort into prettying herself up? It hardly seems to add any further beauty. Even more puzzling is the fact that women who could enjoy a relatively big boost to their looks from these products have almost no interest in them. If a 4 could jump a full point or two by making the same investment as the 9 in make-up, hair styling, flattering clothing, etc., surely we'd expect the 4 to be bonkers about beauty and the 9 to be securely carefree about her looks.
The short answer is that the 9 has a comparative advantage in looks, while the 4 doesn't.
Let's say there are two dimensions that women would compete against each other on, looks and ambition. Guys prefer both because looks signal her genetic quality and ambition means you won't have to spend so much to support her -- you could live in a nicer house, send your kids to more prestigious schools, etc. Blessed by fortune, Riley scores 9 on looks and 7 on ambition, while the less fortunate Mabel scores 4 on looks and 6 on ambition. Clearly every guy would prefer Riley because she scores higher on both things that they care about. But some of them aren't going to have a snowball's chance in hell with her, and they'll listen to Mabel's sales pitch.
Even though Riley could emphasize either trait and demonstrate her superiority, the contrast between her and Mabel is greatest on the dimension of looks. So if she wants a slam dunk, she'll try to compete with Mabel on looks. Mabel recognizes that she's inferior on both dimensions, but she only looks a tiny bit worse on ambition, rather than catastrophically worse on looks. So if she wants to minimize the guys' perception of her inferiority compared to Riley, she'll try to compete based on ambition. Competing on looks requires spending time, money, and effort on beauty products, while competing on ambition requires little such spending.
So we have people who appear to not really need beauty products, from a value-added calculation, and yet who devote their discretionary time, money, and effort to them. Good-looking women buy beauty products not because they get a bigger bang for their buck than do ugly women (it would be the other way around), but because good-looking women are competing based on looks and ugly women are competing on some other dimension where mascara and butt-sculpting jeans are irrelevant.
Wait -- what if Riley was a 9 on looks and a 10 on ambition, and Mabel was a 4 on looks and a 2 on ambition? The gap in looks is the same as before, but now the ambition gap is even larger, so now Riley should want to compete on ambition and Mabel on looks.
But let's remember what the reality is: good-looking people obsess more over their looks than ugly people. That must mean that the real world is more like the first scenario than the second. The reason is probably regression to the mean. If Riley is a 9 on looks, she's very high above the average, so if we look at how she scores on some other trait, it may also be above-average -- because of good genes, a favorable environment, good luck, or whatever -- but it likely won't be as high as the first. Similarly, if Mabel is a 4 on looks, she's extremely below-average, so if we check her on some other trait, it may also be below-average (for the same reasons as before) but it'll probably be not quite so below-average.
Every person is a list of points that show where they score on all the many dimensions of attractiveness. They generally aren't radically different; people above-average in one desirable trait tend to be so in others as well. There's a kind of center of gravity that their various scores hover around. When we notice someone who's a 9 on looks, we don't know where their center of gravity is -- it could be that most of their values hover around 7 or 8, and the 9 is their upper outlier; or it could be that most of their values hover around 10, and the 9 is their lower outlier! But because it's more likely that someone's center of gravity is closer to the population average, rather than farther away, the 9 on looks is more likely to score 7 than 10 on something else like ambition.
For the same reason, it could be that a 4 on looks has a center of gravity that's 6 and looks is their weak suit, or it could be that their center of gravity is actually 2 and looks is their strong suit. Again, we're more likely to observe someone with a center of gravity closer to the population average, so the 4 on looks probably does better on average on other traits.
To try stating it more clearly, if we look at all women who score 9 on looks, many more of them will have their trait-wide average below 9 than above 9. And when we look at all women who score 4 on looks, many more of them will have their trait-wide average above 4 than below 4. That puts us in the first world we looked at, although with some exceptions. There are women who score 8 on looks but 10 on ambition, and we expect them to buck the trend as good-looking women who don't care that much about how they look, but who instead slave night and day to get a leg up on their competition in the job market.
And of course there are the odd women who score 4 on looks but 2 on ambition. These women couldn't care less about their education or career, and even though they have little to show off, they devote most of their time to playing it up -- "I don't care what anybody else says, I know these Juicy track pants make me look good!"
Finally, this applies to men, too: the 9 on looks is more likely than the 4 to use skin moisturizer, wear flattering clothes, and so on, because most good-looking guys score a bit lower on other traits and most ugly guys score a bit higher on other traits. Again, there are exceptional guys who invest more in other pursuits despite being good-looking, as well as the occasional ugly dude who spends all his time showing off his repulsive face and body because that's sadly his strong suit.
Anyway, the take-home message is that people invest in playing up their forte, rather than in the trait that will give them the greatest improvement, to try to shift the competition into an arena where they have a comparative advantage.
There's the further fact that women who enter into the looks-based competition will all be fairly good-looking, so that whatever minuscule improvement they can get from make-up, etc., might be worth the money. But this is just an arms race -- all will buy flattering jeans, wear nice make-up, and so on, so that the ranking among them is largely unaffected by investing in beauty products. If we weren't careful, we might only see this aspect of it and conclude that good-looking women spending so much on make-up was a purely wasteful arms race.
But in reality they're blowing all that time and money in order to shift the competition to one based on looks rather than any of the other dimensions that they could be judged on. That's what gets you access to the top tier of suitors -- you at least get your foot in the door, regardless of how you'll end up relative to the rest of the minority who get their foot in the door.