Why do americans think they are the center of the world?

On the internet it seems like americans often see their way of life as the "standard." You see it when they talk about things like measurement systems, politics, everyday things like tipping in restaurants, translations in foreign media, etc.

I understand if it’s in a forum aimed specifically toward americans, but all over the internet you can see it in global discussions where people from all over the world are participating. There’s often an assumption that everyone follows american norms, and everyhing else is an "exception."