Actually I think it's more that Americans just have a higher cultural tolerance for risk. It takes a certain unusual kind of person to jump off a cliff and try to assemble a plane on the way down, and for some cultural reason, America generates more of those people.

To even have a shot at getting a successful startup off the ground, you need to a assemble a whole team of those people, which is still much more difficult in Europe (though things may be starting to change).