Outraged by the Jan. 6 Capitol riot, a wilderness survival trainer spent years undercover climbing the ranks of right-wing militias. He didn’t tell police or the FBI. He didn’t tell family or friends. The one person he told was a ProPublica reporter.
It's not just that. These people simply take the prevailing ideology pounded into everyone with omnipresent propaganda to one of its logical conclusions.
You can get a lot of people that way, you have a big pool. By contrast to become an organized member of a left-wing political movement you must reject everything you've been taught, the entire mainstream media, the dominant entertainment narrative and begin entertaining explanations for things that leave you alienated from liberals and conservatives if voiced too loudly and openly while these reactionaries can get more and more cooked and extreme and still retain normie Republican friends and relations after expressing all but their most direct agitation for violence type opinions. Simply holding to normal Americans that the US is a force for bad things, does imperialism, shouldn't be supported is a sure-fire way to be cast out as an extremist by co-workers, friends, family, etc. On the other hand the simple reactionary contention that the US government is too woke, that uh they're infringing on individual rights and liberties, that we need some sort of mass reactionary movement to take back our rights will be nodded along by or shrugged off by most Americans.
Add on that most self-aware members of the left are much less likely to have ever been cops or joined the military and the years of experience and network effects of being former either of those don't exist for the left either meaning it's stray individuals who took the time to train or radicalized after leaving the military who have to find each other.
The consequences are also lesser. You won't have any kind of mass movement to free leftists imprisoned on trumped up charges and they'll be gleefully condemned and distanced by liberals and the Democrats who falsely pass themselves off as something resembling left in the US, meanwhile the wider right will immediately rally around anyone but an open neo-Nazi and proclaim the government did them dirty. The feds and local cops see these militias as mostly lost sheep, they have similar politics after all they just think these people take it a bit too far, but at the end of the day they like them are reactionaries at heart so there's less interest in pressure there.