There is an old, I think Chinese, cliche: "A good sword is a terrible thing. It begs to be used."
The US has so much military, political, economic, and technological power. These powers beg to be used.
But because the US population isn't entirely stupid, something is needed to justify creating and maintaining that power. So the weapon doesn't just beg to be used, it HAS to be used.
Look at the "war" on drugs, "war" on communism, "war" on terrorism, and now a "war" in cyberspace. How better to justify all that power than to find something, real or imagined, to use it on? And how better to continue justifying having that power using it than to pick an "enemy" that is so nebulous and difficult to define that the "war" never ends?