I think at this point it's signaling. The main point isn't to insult the person, the point is to signal to women that "you should not hang around this person, have a relationship, or sleep with them".
(The assumption is that their side has a monopoly on promiscuous women, which is... well, a topic on its own.)
If you call someone a racist or fascist, you make them "the bad boy", and potentially attractive. If you call them an anti-woman creep who isn't a good lay, you're trying to counter-act that effect. And if you call a celebrity an incel, you're really calling their fanbase incels, hoping it scares away women and the men who want to sleep with them.
It's a decent branding attempt, as far as it goes, but it got overused too fast.