What about travelling inside of the US? Is that not travelling?
I think that's part of it too. The US is so large and diverse, that you could go on a vacation every year to a different place and never leave the country. I've always used this as the main reason I don't vacation abroad. There are dozens of places in the US I'd like to see and I can go to them for less than half the cost of a trip abroad. I would imagine that many have the same opinion as me. And most Americans can't really afford a trip abroad.
Of course that doesn't explain why we aren't well educated on other cultures.
I think it's also important to point out that the US is massive. So, so massive. It spans more time zones than the whole of Europe, probably, I'm guessing. (And I mean geographically, as opposed to culturally, where most of Europe's mutually decided they want to be an hour ahead of GMT regardless of where they are in the atlas.)
I live on a big wet tea-slathered rock just north of France, so I only have to move my head eight degrees and everyone's speaking prussian. I've travelled to Spain twice, and Italy once, which isn't a staggering list, but in the UK it makes me "travelled." Check the distance on Google Maps, though, and
England to Spain is 1,155 miles by car. And it'd be far less as the crow flies. That's about what, one time zone? Poxy. You could drive 1,155 miles and never leave Alaska in the US.
The drive from my hometown to Venice, meanwhile, would've been 1,017 miles had I not planed it. That's fifty miles
shorter than Mount St. Helens to Las Vegas. In the UK, that trip makes me cultured, 'cos I go through border control and own a passport. If I made the same trip in the US, I'd be a luddite.
So frankly, I'd argue the main reason America's considered so isolated is because you're being held to much higher standards. There's every chance that you, the American reader, having never left your country, have possibly seen more geographical diversity than I will in my lifetime. So do feel free to tell any smug Brits who tell you your country's thick that they can fuck off. From me. Personally.