It is a byproduct of gender mystiques. If we go back to when The Feminine Mystique was published, the Problem with No Name was that women felt as though they couldn't possibly be women because they were so unlike the depictions of women in magazines, who found deep contentment out of vacuum cleaners, always looked glamorous even when exhausted. Everything that brought them personal joy was deeply weird by magazine standards, and they had no outlet. There were even incels back then, as a common piece of advice was that all a woman's problems could be solved through the fulfillment of being a wife and mother.

Incels today are often men who are gender-nonconforming, even when they try to compensate. Reddit picked up on this by naming them neckbeards -- even when they try to conform to what they think are the gender norms, they fail somehow, like the beard on their face. Without the language to process these feelings, they are stuck in the same loop as the women before Betty Friedan published The Feminine Mystique.

The political activism is a way of trying to escape these feelings of frustration. This is actually something that is done by others as well -- a good book on the subject is The True Believer by Eric Hoffer, who noticed the same thing studying who became a Nazi and who became a Communist in the 1940s, and realizing it was the same type of person, someone struggling through personal failure who wanted a cause that would let them escape from themselves.