I'm American, and I lived in Moscow for 4 years, and recently spent 5 months backpacking through Asia, so I don't fit the stereotype.
I don't think distance or cost are sufficient to explain Americans' reticence to travel. Australians live in the back of beyond - it totally takes like 115 hours minimum for them to fly to Europe - yet wherever you travel, there you will meet Aussies.
I think it comes down to fear, and lack of interest, and lack of time.
During my time abroad, I never once experienced anti-Americanism, yet to listen to people here back in 2003, you'd think it was a rampant problem. And many people can't understand how it's possible to travel in countries where you don't speak the language.
Then there's the lack of interest among many Americans. If you're into the outdoors, as Rockstar appears to be, then overseas travel might not hold the same appeal, and that's fine if that's where you're interests are. I would say, though, that even Americans who would be more interested in tourism to historical sites, or examples of fine architecture, or great cities, will still usually prefer to visit their homegrown versions. Which is a shame because in these areas the US just can't compete.
Finally, we don't get enough time off here. There is no federal law which mandates any right to vacation, and employers can make whatever policies they like. Any such law would be Socialism, and therefore Bad.
