In all honesty, you could fit all of the European Union in the footprint of the USA twice with room to spare. And until recently there was no need for a passport to travel to Canada or Mexico as a US citizen. Oregon is about as big as Germany, IIRC, so traveling ANYWHERE in Europe requires a passport. The USA (and, until recently, Mexico and Canada) has so much to offer and is so big lots of Americans do a significant amount of travel without one.
I'm not belittling your point at all, as I have traveled through most of Europe, visited a little of Canada, and been in at least 2/3 or so of the USA - I fully recognize the benefits of travel, including international/foreign travel, to the well-roundedness of a person. But in CA alone you can go from ocean to desert to giant redwoods to vast farmlands and never leave the state. Heck, the lowest spot and the highest spots in the contiguous 48 states are just about 2.5 hours apart by car and both are in California! So, at least from the point of view of visiting different environments, the US has a lot to offer anyone who lives here, even within the same state. If you throw in the folks in Minnesota vs. Alabama vs. Oklahoma vs. New York vs. San Francisco/Seattle vs. wherever, you can also experience quite a bit of native and international culture as well.
Just wanted to throw some perspective in on the need for passports and travel. Didn't mean to throw in a tangent. Carry on.