Yeah, femcels is not a real thing, you can even find good looking Incels, but you can't find genuine femcels.
While it's true that a hot girl will have a higher smv than an ugly fat one; while it's true that a beautiful woman will have a happier and more sucessful life than an ugly one, that is not the same as these genetically disavantaged women having 0 smv, nope, they still have a higher smv than the average joe.
Why is that? I think it's mix of genetic and cultural aspects. I'll try to summarize them:
1 - Genetic. Men have a higher sexual drive than women; men can impregnate thousands of women per day as long his dick is working properly, as long as he has acess to sex; with women is different, women not only have a lower sex drive, but they can only get pregnant at once, save for rare occasions. I think our high sex drive makes us kind of blindly to the female imperfections, that explains why so many men find ugly women hot, because our sexual appetite is higher, if you're starving even the worst food taste like heaven. So for a woman to have sex, she only needs to breathe/exist and a man is in her field of view.
2 - Cultural. Society had always glorified womanizers/slayers, while putting female beauty on a pedestal, there are quotes saying all women are beautiful, amazing, but the opposite, regarding men, well, they are nonexistent, or not as a common. Since men are expected to slay, if not you're seeing as gay, since women have an intrinsic value that men lack, the result is: an entire gender (women) where incels are nonexistent and one (men) where they are numerous.