Why is American football called the world champions when it only played in USA and only played by Americans?

Why do Americans during the superbowl say "world champions" to the winner team when the Competition is only held in USA. I don't understand World Champions should be referred to a Competition held globally like the Olympics and World Cup. Same goes for "World series baseball". Why would Americans say this in a national competition? i mean you "World Champions" should represent a country that won a sport championship over other countries competing. or is it just to pretend around so the popular sport in the nation could be considered as global?