How do people in this day in age become nazis/neonazies sexist or even incels when there is so much knowledge against it? Do they get anything out of being that way?

Nazis and incels need to be dealt with, yes, but the important thing to keep in mind is they are symptomatic of suffering swaths of the population. People don't just do hate because it's fun.

We let big businesses and the rich steamroll entire communities and industries, pay lip service to helping people who've been damaged by capitalism, and then after the election cycles are over leave their communities to rot. They are desperate and turn to the wrong answers because there aren't any others.

We allow entertainment and advertising to blast our society with a particular view of what relationship "success" is, and accept mockery of those who cannot thrive in that narrow definition due to social anxiety or other mental issues as fair game. Those men are desperate and turn to the wrong answers because there aren't any others.

Yes, Nazis and incels are absolutely awful, hateful problems that must be dealt with. And by the time they reach that point, I'd argue they probably can't be saved. But they don't fall out of the sky. They come from normal people whose cries for help went unheard, sometimes for decades, or generations. They're the product of systemic injustices that we can mitigate with outreach programs and getting serious about mitigating the social problems that create the soil they spring from. Stopping them is a necessary band-aid, but the real solution is to address the situations that allow them to thrive in the first place.