The US still has a frontier mentality. It's embedded in our culture, and in the case of groups like the Mormons, it's actually part of their religious obligations[1]. If you've been outside of the coastal urban cores, you can understand why: the place is huge, mostly empty, and most of it, the climate can be extremely hostile (compared to, say, central or southern Europe -northern Europe actually has something like US prepper culture). The US also has racial fault lines which have periodically broken out into civil insurrections and outright warfare; the last Indian wars were in the 1920s [2], and inner cities periodically break out into riots. It's fairly well known that general social trust in the US is on a precipitous decline, which of course encourages this sort of thing.

A consequence of the size and frontier mentality: the US also has a huge hunting and gun culture. Tens of millions of people actually hunt, and many tens of millions more people have hunting family and friends or do similar outdoors activities that encourage the "prepper" mentality of being able to survive in the wild without any help from civilization. Western Europe, for contrast, has a comparable population and about a third the number of hunters, and the history of hunting in Europe (maybe aside from France) is more of an upper class game preserve thing than a frontier thing.

Americans also have a fundamental mistrust of government. Lots of fairly rational reasons for this. Many people live fairly far away from any real government officialdom. You aren't far from police or firemen or animal control in a big city. If you live in the countryside, believe it or not, coyotes might eat your children. A guy 100 miles from me, in a fairly urbanized area, had to strangle a coyote who was trying to eat his kid[3] just the other day. Also, generally speaking, the US civil services do not have a reputation for diligence or efficiency. The types of people who are into prepper mentality generally falls into the set of people who has profound mistrust for government's ability to provide any social services efficiently in event of a disaster or in any other situation.

For recent history, various civil defense initiatives have also encouraged the tendency: in the 1950s peak cold war, many otherwise normal people built nuclear fallout shelters in their back yards, encouraged by the government and the media. We don't have those cool centrally planned underground parking garages the Europeans built to deal with this sort of thing. Ham radio is also a part of all this: there are nearly three quarter of a million active amateur radio licensees in the US, and all receive some formal training in disaster preparedness.

"Preppers" are a cultural part of all these tendencies. Maybe their preparations seem silly or misguided, but it's deeply American in the same way the Blues, free speech absolutism and PTA meetings are deeply American. I suspect it will fade with increasing urbanization and social pressures, but it will always be there in some form. Heck, we even have chain stores which cater to this outdoors/survivalist mentality[4]. I myself am half redneck and kept a bug out bag ready when I lived in Berkeley (an earthquake zone). I wasn't envisioning holding off hordes of cannibal hippies after my canned goods; more like hiking into the woods and living off pemmican and the local animal population for a week or two until the shortwave said to come back. There are a few recent history events which seem to justify this sort of thing; floods where city officials fled and fishermen provided emergency services[5].

[1] https://mormonrules.com/list/mormon-rules-emergency-prepared...

[2] https://en.wikipedia.org/wiki/American_Indian_Wars#Last_conf...

[3] https://abcnews.go.com/US/hampshire-father-strangles-coyote-...

[4] https://www.basspro.com/

[5] https://en.wikipedia.org/wiki/Cajun_Navy