Most of the world's best baseball players play here
Baseball is played in several countries, but most of the best players play in America (there are some damned good Cuban players but Castro keeps them at home). Look at the major league rosters and you'll find players from countries like Honduras, Cuba (a few did get out), Japan, and several others.
It's pretty much the same with several other sports. American style football is played in a few other countries, but most of the best players are here. The same is true for basketball and hockey (except for the lockout this season). The reason is pretty basic - a top notch athlete can make millions here in those sports because they're popular with American audiences.