They don't have that much power. It takes a lot (and a lack of sanity) for someone to "kill a bunch of people on their way out".

The most they can do is something like Jan 6th with more competence. Something like that requires organization, and the FBI is pretty good at infiltrating organizations who are trying to grow enough to make a difference. January 6th wasn't stopped primarily because it was done by the President, and the office (for better or worse) gets more leeway with laws.

Seems likely, their stupidity functions as a safety mechanism. It's just like why elaborate government conspiracies fail: People are eventually stupid and untrustworthy, the more people involved, the greater the likelihood somebody fucks up hard. When you take a group that's already very poorly educated and lacks critical thinking, they're probably not going to do too hot in any form of sustained warfare.

Doesn't help that they think people who aren't far right are unarmed soy boys: There are plenty of moderates and liberals who are armed and can fight just fine.