Because it is fashionable to pile on the US. The media, the middle east, europe, hell even our own president, loves to talk about how bad we are. We are told to be respectful and tolerant of these people when they slap us in the face with their snide remarks because we have become a feminized nation. Males are told the bigger man will walk away from a fight instead of defending himself.
Also people are told to hate success. America is a Cinderella story. Starting out as a small, handfull of colonies and growing to a superpower in a small amount of time. This happens with companies too. How many people do you hear talk about how evil Walmart is ? Even though, like other companies, they started out as a single small store. Then through smart decisions and investing in themselves they achieved the American dream. I guess many people think they should have only grown to partial success and then stopped growing.