Why don't more people travel from the USA to Europe the way Europeans travel to the USA?

I lived in Germany for a year and traveled through Europe several times with ease. My friends back in the USA rarely go beyond the USA and Mexico. I tried to convince my Dad to come over to Italy for a week and he thought it was the craziest thing he ever heard. What is it about Europeans that makes them want to travel more than Americans to Europe? (Obviously I know there are many Americans that take vacations to Europe but not nearly as many as there are Europeans that vacation in the USA)<br><br>Edit: thank you all for the replies so far. Also just to clarify this was meant to be people crossing the Atlantic I don't have a problem with Americans traveling in the USA