You're right that there is a very low probability that we will face a scenario that requires guerrilla tactics and heavy weapons, but keep in mind that people aren't necessarily wary of foreign powers invading, but our own government attempting to strip us of our rights.
A lot of people don't trust our own government, and different people show it in different ways. Some choose to protest, some are members of movements, while others choose to be prepared when the government turns on its citizens.
Back in the 1700s most of Colonial America was populated by very independent outdoorsmen who knew how to hunt, which translated to effectiveness on the battlefield and cultivated the warrior spirit. Today, much of that ethos is lost, but the few who retain the same ideals as the militia back then prepare on their own time and use their own money. I forget who said it but it was like, "People should not be afraid of their government, the government should be afraid of its people."
edited to add: Thomas Jefferson said it.