A large part of the FBI's job is to shut these militias down. You can't grow a resistance movement without making some noise, and the US government is obviously very keen on maintaining the status quo.

Examples from the past are easy to come by (COINTELPRO), but a recent example would be the failed 2020 kidnapping plot of Gretchen Whitmer, the Governor of Michigan at the time.

Thirteen arrests were announced, and the FBI has admitted to using three informants and two special agents. The defense argued that there were at least twelve.

Using the official number as a conservative guess, that's still 5 feds for 13 arrests (a 38% ratio!)

Now imagine if instead of 13 dudes trying to kidnap a governor, it was a local militia trying to arm and train hundreds or thousands of people. The full power of the US government to crush opposition is terrifying.