For all but a tiny fraction of Americans, the cost of a passport is a tiny, rounding error expense compared to actually leaving the country. This isn't Europe, where you take a wrong turn and end up in a different country. Here in California, there's a highway you can drive on for 750 miles and not even have left the state (like driving from Paris to Warsaw). And we're just one state of 50. On the diagonal, crossing the continental US is like driving from London to Tel-Aviv.

Nearby, we've got Canada and Mexico, and up until pretty recently, you could cross over those borders with a driver's license. And both those countries are big. On the other sides we have oceans. So for most Americans, the minimum cost of an international flight is the same as the cost for a European to fly to the US ($500-$1000), and a full day's travel each way. Here on HN, we might forget that most of the population makes fucking peanuts, so keep in mind that means that for most Americans, $1000 is a lot of money. Most Americans also don't get a lot of time off, so those 2 days of travel are a significant cost in of themselves.

All told, the lack of passports amongst Americans isn't indicative of some isolationist mindset. It's just that they have no need of a passport, because they aren't taking the kinds of extremely far-flung vacations that would need one, and they know if they need one, they can just get one before their trip.