Why do all sports in USA say they are world champions?

are they saying they are better than anybody else in the world?? Baseball, World Series champions, Basketball "World Champions" same goes with the MLS. in Football/Soccer Im 100% percent sure that there is way better teams and leagues like the Spanish,English,Italian etc... and in Basketball and Baseball, how are they sure they are the best??? have they played every single team in the world? or what? I remember Barcelona beat the Lakers last year. it was a friendly but still. and by the way im half American, I love this 3 sports but i just think it's kinda dumb that they call them selfs the best in the world.