because no other country can be bothered to play American football
Because no other country can compete with NFL teams in American Football. The same reason the World Series champion is called the "World Champion" (though, in the case of MLB, there is one team, the Toronto Blue Jays, outside of the US).
Moreover, the title "National Champion" is used by the champion of College Football, and is thus unavailable for the champion of professional football.
World Cup you mean? Italy.
Christians Ronaldo (Real Madrid)
Italy is the reigning world cup holders.
Spain National Football Team.
Brazil is the record World Champion with five world cup victories.
There is no American Football around the world because it is not popular outside the US
it's all about sports and handballing and kicking and get a goal and be the world's best football champion