Why join? A need to belong. The thrill of being righteous. The chance to act out pack behaviors on some terrified victim who has strayed to the wrong part of town.
Maybe you weren't cool before. Now you can be cool. You're a secret agent for good against the Evil Dumb people. (There's generally a conspiracy of Evil Dumb People opposing the cause.) No matter what you have to do to win, all the good people on your side already know you're doing the right thing. That's not even at issue.
Best of all, because you're allowed to filter your inputs to keep out all the propaganda from the Evil Dumb side, you never have to face being wrong.
It basically is a delusional shell comparable to a mild psychosis. Nothing's perfect, but the people who hide within such a thing in the first place for any length of time tend to get pretty comfortable in there. I don't think that many of them are reachable.
Does "delusional shell" have a technical meaning? It's a very evocative phrase.