Because they play with themselves.
1. World series: Baseball. There are no international teams (maybe from Canada, but certainly not representative of the world.)
2. Superbowl champion: Is regularly referred to as world champion. There is literally no team from outside the US playing in NFL. Not even all states have their teams.
If you play with yourself, you will ALWAYS win.
PS: Any Americans please don't be offended. It is a joke.