The other thing about EU is the buildings were/are built to last....and the walls are so thick, it was easy to figure out how to hide things in them, under cellars, in caves, etc. etc.
In the USA, most of our homes are made of 2x4s and plywood.... a modern day occupier with thermal gear would ferret out stuff unless it was literally buried so deep as to be virtually unaccessible without alot of digging....
Then too, most Europeans were townfolk, cityfolk..... or lived in the boonies. They all knew each other going back generations so it was easy to help each other....most of our families are scattered, we don't really know each other very well.... neighbors are anonymous, etc.
But one thing we have going for us is the sheer size of these United States.... that people with out advanced degrees have been able to successfully carry on a multi-billion dollar trade in illegal drugs for the last 30 years with every jurisdiction after them.... tells me some times sheer size and extension is the best defense. Individual drug smugglers/producers/distributors routinely get taken down...but so long as there are tens of millions of clients, there's an inexhaustible supply of 'new' people to 'provide the service'.
In a situation of occupation....while we don't have the heavy weapons caches all over the place, I think we'd see similar attrition rates going down, drip, drip, drip from good ol boys taking odd pot shots with their 30-06s every now and then.