Long story short, long long ago, men who today would be thought of as "incel looking" men set up social policies in order to stop women from being monopolized by men who today would be referred to as "chads" or "male models".
That social policy was monogamy.
The men in power, most of whom were ugly, needed this, to get women to sleep with and procreate with them despite having no real attraction to them, so they convinced the female population that they would be punished by God if they did not comply with their demands. They knew that the women wouldn't comply if they weren't getting something out of it, so they promised them resources and stability.
But women are smart and are more than capable of getting that on their own. So these men made them second class citizens, and convinced them that that's how God wanted things.
This worked for centuries, until the sexual revolution came along. And now that women are more self sufficient, they can afford to focus more and more on the men they're actually attracted to. This leaves men who may have benefitted from the old system in the dust.
But precisely WHY are there still so many incel looking men today when evolution should have wiped them out long ago? Because the social policies your ugly ancestors wrote helped keep them in the gene pool. But you can't repress anyone forever, and with repression of women ending, the descendents of the ugly men are bearing the brunt.
This is another example of how when you repress someone, you get EXACTLY the opposite of what you originally intended.
But the good news, in the long term sense, is that today's day and age reflects the natural order of things. Incel genes will further and further die off, people will start realizing how important looks are, and the system will recalibrate itself. In 100 years, almost no guys will be incel, because most guys will have genetics that will enable them to pass most womens' looks thresholds.
The causes of incel problems aren't women. It's that incel men of the ancient times wanted to repress everything.
If every man in the world was a Chad, then women would view the men who are lesser Chads with just as much contempt as they view ugly men in modern society.
If a girl grows up in a remote village with no other guys around except an ugly guy, she will fall in love to him.
Girls are sexually attracted only to their most attractive option. What matters is not your absolute attractiveness, but your attractiveness relative to the girl's other options.