Well, I'm a lifelong traveler because that's how I was brought up. Egypt, Spain, Iceland...also went to Peru with just my younger brother (no tour group or anything, just me and my high school Spanish) when we were in our teens.
Most of the kids I went to school with considered driving to a nearby county a "trip." Flying on a plane was a huge deal to them. Not surprisingly, kids who grow up like that don't know what the world is like, have no appreciation for different cultures beyond those funny hats people in their history book wear, and in general are less educated. No matter how much you know, if you've lived in just one country all your life, you don't know crap.
As for why, well, distance plays a role, as does money. But neither of those are very important. My brother and I ate on $6 a day in Peru- three full and delicious three-course meals for each of us. You don't have to go to London to be abroad. Here's the real reason Americans don't travel: their parents didn't raise them to be adventurous, to question, or to wonder. American exceptionalism is a big part of it, too. If we just KNOW America is head and shoulders above the rest, well, you'll just be polluting yourself by visiting a foreign country, now wouldn't you?
Sad.