Ha Fraulein, you reminded me of something I haven't thought of in a while. When I came home from England, I noticed after a few weeks that people on the street were looking at me as if I were strange. I couldn't figure it out until I realized that my saying, Hiya, or good morning or whatever to everyone I passed on the street was freaking them out. After 18 months in England, it seemed perfectly normal to me!
I heard a Professor from Columbia University, New York, give a lecture here in UK and he claimed that only (IIRC) 5% of US citizens owned a passport and that even amongst congressmen the figure was only 30%. His point was, of course, that most US citizens do not travel abroad and so had little knowledge and little experience of other cultures. I don't know how accurate his figures were but, assuming there is some truth in what he claimed, why do so many US citizens not travel more?
The first obvious answer is cost. In England, buying a return ticket to Germany or Italy is like me flying from LA to Chicago or closer. Second, I think it's a bit of a stretch to equate a drunken weekend in Ibiza with any knowledge of international relations or culture. I may be out of the ordinary (but I doubt it) and offhand, I can think of friends and relatives that have lived in or visited; Japan, Taiwan, Mexico, Brazil, Russia, Argentina, Uruguay, Australia, Korea, Honduras, UK, France, Germany, Switzerland, Finland, Norway, South Africa, Zimbabwe, Liberia, Israel, Tahiti, Romania, and the list goes on. Again, I don't think that makes any of them experts on the countries they've visited, but I don't think Americans are generally any more insular than citizens of other countries.
Granted (as John mentions above), many of those trips were missionary related but not all of them were, and a great deal of those people ended up going back many times after that (like me). I think part of that is that once you've crossed the threshold and seen it's not that hard to visit other parts of the world, it's easier to do it again.