Beauty carries diminishing returns for women.
Discussion
On a scale of 1-10,
-- women are usually desperate to make it into the very comfortable 4-6 range
-- 7-8 range is a nice bump
-- 9+ is usually no advantage over improving in other aspects.
And women usually have a range of ±1 or 2 that they can calibrate.
Ideal is probably a 6, as she can pull out all the stops and squeak past a 7 or even an 8, but can go unnoticed and blend in, if she wants to.
It likely to be true for any "property," e.g., for physical strengths, it not great to be weak, say below 5, it's pretty great to be string, 7-8, but super strong, 9+ takes inproportional amount of effort and is simply not worth it. Same goes for apartment cleaning, I like the 7-8 area, but would not really want to do or even live in the sterile 9+.
Achieving and maintaining beauty takes effort, so women have to determine how much effort to put into it, as opposed to putting the effort in elsewhere. The better-looking they are, the less profit they have from becoming even prettier.
And being "good looking" can carry a cost.
That's why women who work in male-dominated or intellectually-challenging industries often take pains to "dress down" and appear a bit nondescript.
Unlike women outside of those industries, being less overtly attractive doesn't cost them in the romantic field because they tend to have regular and intimate access to successful men through their workplace. They can actually hurt their chances with those men by looking "too hot".