[Zhang Jie]

The reason why people who can't build relationships go incel modes is the shame of their inabilities to build it and the reason why they feel ashamed is that the society judges people by their sex appeals.

Maybe it is a pretty common knowledge but we have to fumble out the science behind this phenomena.

My guess is that in the primitive society the demographics is an extremely indication of how much sovereignty a horde can acheive by wars since as we all know, the more people in your horde, the stronger your horde. And it is impossible to produce many children with only a female, so 'polygamy' is necessitated. And how much sex appeal you have was progressively becoming an indication of your personal value as the society was evolving.

[Ren]

I think they weren't so smart during that "horde period" to plan how many kids they'll have. I think they just followed their libidos and everyone who could just had 1+ females. Like the situation in the present apes social structures.

[MSL]

 It's an important factor indeed but mostly and firstly the so-called "society" (in fact, just the majority of the individuals) evaluates people by their capital (property, money) and power. In China, for example, a person like Jack Ma (马云) who is one of the richest in the world will be evaluated higher than some handsome males like TFBoys (for example -- Wang Yuan (王源 from TFBoys)).
 Maybe, there are 30%-40% of the teen-girls and the younger women who will still think that Wang Yuan is better than Jack Ma but most of the mature young women, the elder women and most of the males will do agree that Jack Ma is better (and if there is a choice, "You can marry only someone like Jack Ma or someone like Wang Yuan!", I bet Jack Ma is going to win like 60:40 or 70:30.)
 For the non-Chinese readers and those who know better the Western world, just change the people in this example: Jack Ma - Elon Musk and Wang Yuan - Justin Bieber. Most of the mature males and females and some of the practical young females will choose Elon Musk not Justin Bieber.

[Alexa]

"There are incels because the society evaluates people by their sex appeal" you said. I'd like to add "There are incels because they haven't money at least to beta-buxx".