It’s a damn big country. Logistics are against something like that.
The other problem is the economic one. It takes an economy to support the military. Cutting off a good part of the country works against that.
Per the FBI video of Larry Grathwol (the only FBI agent to infiltrate the Weather Underground), the Marxists would invite our enemies to help them secure the country.
Pootie Poot would be glad to help!