That would be a hard pick for me, the former or latter.
IMO Americans are fed a constant diet of skewed and filtered information about the outside world that holds very little resemblance to what is actually happening out here. The first reaction/question out of most Americans about our travels relates to fear. I grew up being curious about the world but today, it's hard to find an American who hasn't been taught, and isn't reminded on the news every day, to fear the world.
Europeans have a long cultural history of exploring, discovering and subsequently raping and pillaging, the known (and unknown) world. Consequently, many of their educational themes, literature, heroes and traditions include global aspects. While America's adventures were in the wild west, Europe's were in Africa, India, Asia and everywhere in between. When Americans think of adventure, they think of Moab or some other domestic destination. For Europeans, entry level adventure is Africa.
So, take the two factors together and you've got 300 million Americans afraid to go outside their homes, much less across the border, while you've got 490 million Europeans all aspiring to drive overland to the Cape of Good Hope or Beijing.