It's the same mentality that gets people dragged into cults. People aren't crazy kool aid drinkers on the first day. It takes time, where at first they dismantle any sort of critical thinking. This is "listen and believe", or "white allies need to just shut up and listen". It's to get people not to actually think about these things, usually by triggering an emotional response. You want to be a good ally, right? A person who uses critical thinking will realize that it's much more likely that someone is being paranoid than that everyone else around them is secretly sexist/racist.
Then they try to cut out anyone or anything that's "problematic". This does two things:
1. Socially isolate the person from non cult members. They start to believe that the cult is the norm. It also makes it harder to leave the cult, because they've cut off all ties to anyone outside, so they would be alone in the world if they leave.
2. Isolate them from conflicting information. They only allow information in from "good" sources. Dissenting opinions are evil and not to be believed.
Once all this has happened, it becomes much harder to change their opinions. Doing so not only means having to admit to making a massive fuck up, it means having to give up most of their friends. It also means potentially having to come crawling back to people they've probably burned their bridges with. Some of them are not even on speaking terms with their own families. To come back and ask forgiveness would be one of the most humiliating things they could possibly think of, so they'll go out of their way to avoid that. It's a lot easier to interpret any true statements that conflict with the narrative as lies that evil people propagate to keep the outside world evil and against them, and that their group is the special enlightened ones that know the true and honest truth of it all.