Women know that if they get involved with a male that's lower in sexual marketplace value, that male will know he's below her, and thus will attempt to do all he can to keep her, because his value as a male is tied to his relationship status, particularly his relationship status with the "more attractive" woman in question.

These are the men that wine and dine their women while their women provide them with nothing. The ones who carry multitudes of shopping bags (full of things they've bought with their money, for their woman), who essentially base their entire life around being of service to their woman. They are slaves to pussy. And they know this. And they know that at some point their woman will leave them for a "better offer", which is why they do this. They know all they have going for them is their utility value. So they attempt to pour resources into a woman as though she's some sort of purely financial investment in the desperate hope that this will make her stay.

Another telling commonality in these relationships is the kind of "couples" photos these kinds of couples take and the fact that usually when these dynamics are at play the male is behind the female with his arms around her, looking at her or kissing her, while she's looking at the camera, with absolutely no attention on him. This is symbolic of who has the power in the relationship. It's called "mate-guarding" and whoever is doing the mate-guarding is the submissive partner in the relationship and the one with the lower sexual marketplace value. The one with the higher sexual markerplace value won't tend to mate-guard because they know that they have nothing to worry about in relation to securing their place in their relationship, because they know they've punched so far below their weight, that the person they're with won't find a better offer.

A lot of old photos of couples in and around wartime for example are of "manly" men standing or sitting with "manly" poses while their women have their arms wrapped around them looking at them with a big smile on their face as if to say "look at the great man I've gotten myself", but now the dynamic has flipped and a lot of supposed men have almost no masculinity inherent to them and as such, women have taken the dominant role.

TLDR: attractive women do this to have absolute power over the men they're with because it provides them with a secure resource of material wealth, possessions, attention and validation.