DHS does put out bulletins about this fairly regularly, and both they and the FBI actively monitor extremist groups. Thing is, most prosecutions you see fall into a few categories:
- Guys selling silencers or full-auto gun parts make easy money and get overconfident
- angry guy who's had enough goes around shooting his mouth off about it being time to 'Do something' and makes a new friend who wants to help but turns out to be an FBI agent
- wannabe guerrilla gets tired of never doing anything besides plinking in the woods and goes on a shooting spree instead (not being flippant; I know of one guy who picked a date, planned for months to shoot up a school, then said 'fuck it' a week early and shot up a supermarket instead because he was too excited to wait)
In some countries people get hit with severe charges for incitement or distributing extremist propaganda or possessing instructional materials (the UK in particular, because they have given their police vast powers to deal with terrorism going back to the 1970s). In the US distributing such material is protected by the first amendment but is a good way to get put on a list and maybe have men in suits show up at your home for some friendly conversation about who much you wouldn't like federal prison.
But although many hotheads signal their intentions loudly enough to get caught in advance, there's not that much you can do about determined actors who are smart enough to think about keeping their digital footprint small and their mouths shut.