Here's the reason why:
Death by Government
History is replete with people fighting against the scourge of totalitarianism - and often losing. In a less technological age, what happened on the other side of the world was a local matter. Given that we do have a global economy and interdependence, totalitarianism threatens the world with a new Dark Age. This is why winning the war on terror is so critical. Civilization is under assault by medieval barbarians. If America withdraws into isolation, we may stave off the impact to our own society by a matter of years, but eventually the Dark Age will engulf us as well.