First off, it's much easier for Europeans to visit neighboring countries than it is for Americans. That's probably 90% of the battle right there. All the countries of Europe can fit into the United States east of the Mississippi River, plus Texas. Traveling to different countries in Europe is like traveling state-to-state for us -- without lengthy and costly airfare.

The UK is slightly smaller than Oregon, Spain is slightly more than twice the size of Oregon, the Netherlands are slightly less than twice the size of New Jersey, etc. Hell, the U.S. is even larger than Australia. Having not to "even been to half" of the U.S. states would be like not having "even been to all" countries of Europe plus most of the Middle East.

Whether it's "not part of the plan" for Americans to travel overseas I'm not sure about -- that's conjecture on your part. I don't know if it's "the mentality" of Americans or not. I'd love to go back to Italy and Costa Rica myself. As well as return to our neighbors to the north and south (Canada and Mexico). And Asia! I also think it is the expense, time commitment, and language barriers as well that may prevent some people from travelling as much overseas as they'd like.