My seven year old watched Pearl Harbor yesterday and at the end asked that question.
After stumbling around trying to explain I got that gaze that only a bright seven year old can deliver; the one that says 'I aint buying that crap - you really don't know'.
Basically I explained that many of Europe's brightest entrepreneurs came to America and flourished in an environment of zero taxes, near zero government interference and a working class that had strong work ethic. And that the US was insulated from homeland attacks in both WW1 and WWII by two large oceans. That the US benefited from the mass destruction of war in Europe and Asia. Once WWll ended US had a giant head start because besides Pearl Harbor the US did not have to spend great fortunes of both time and public money rebuilding infrastructure.
On second thought, maybe I got that glazed look because my explanation was too long?