Because all that shyt has one thing in common: upset men who feel the world owes them
Yeah, and thats not really the only overlap either.
It's kind of an easy pivot to go from manosphere shyt into conservative misogyny etc etc. They are REALLY fukking close together.
It's possible to have one without the other, but its going to be pretty rare. There's plenty of men who work on themselves, their character, their careers and self improvement, without veering into the hard right wing hateful shyt. They probably just don't bother labeling themselves "red pill" because of the baggage those terms come with due to the right wing machine co-opting it.
It is what it is.