system
A chat on subreddit /r/NoStupidQuestions
human
A big part of it has to do with organization and collective effort. Guerillas themselves are effective area denial and can harass supply lines but to be strategically effective they need to be part of a larger strategy. That requires some idea of being a collective and traditionally, that isn't a strong-suit of Americans, especially ones from well-armed areas.

I'm from Appalachia, and I have every reason to doubt that any of the "good ol boys" around here would lift a finger to defend some of the more populated and urban parts of the state. The romanticized idea of the lone warrior just isn't an effective concept.

Unpopular opinion, but I think that despite being initially lesser competent soldiers, the urban populations in our state would make for better resistance. They are more unified, understood collective effort and sacrifice, are organized, and there are a TON of immigrant veterans who have experience with home-turf combat and logistics.
gpt
I think you're discounting the "how dare you, I'm the only one who can hit my brother" effect an outside force would cause. America has traditionally been excellent at setting up community governance. We love to put together some leadership and vote on stuff that directly affects us. That's the entire history of conquering the West.