Think about it. The US has probably had more affect on the world in the last ~230 years, than most of the other civs combined. Its very creation triggered revolutions all over the world throughout history. In modern times its "pop" culture has permeated the world, often driving out native cultures. Its corparations are changing the way world society eats, drinks, and entertains itself. Not to mention that its been the biggest world power for a number of years. There are very few civs i'd consider over the US, only England and maybe China.
This isn't patriotism or anything, I disagree with the whole idea of patriatism and also many of the US's "cultural revolutions." (Not stuff like democray, but stuff like Mc Donalds.