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I disagree about the entire time and money argument. Though it has definite merit, it doesn't answer for everything by a long shot.
I could mention how odd it seems that for many, a vacation abroad consists of traveling on a huge ship and opting for mini-forays into carefully choreographed and pre-vetted spots in "foreign lands".
But I think the major factor is that, in my experience, a majority of Americans seem to have little or no intellectual curiosity about ... well ... the rest of the world, except as it relates directly to them. A core reason for travel, as I see it, is to step outside one's self, outside your usual experiences. I never hear someone say, "I wonder what a person in [other country] thinks about [whatever]".
As another responder brought to mind, I think the furthest most of my co-citizens want to venture is to a theme park. If travel were much cheaper, I don't think it would make a difference -- as I referenced above as to the *way* many of us choose to travel, in little bubbles of familiarity.