Ahem.. coming from a somewhat larger country, I'm not sure that semi-vast distances explain much here..

EDIT:

It is more just the sheer isolated nature of the USA. The country borders on conquered Mexico to the south, and Canada to the north. And since the USA has considerably more soldiers and equipment than either of their neighbours, they generally just get to play bully in the sandbox, without worrying what anyone else thinks or cares. All of the countries populated enough to shove back are simply far enough away not to matter to them.

Cheer