The National Football League is the dominant football league in the world. Major League Baseball is the dominant baseball league in the world. The National Basketball Association is the dominant basketball league in the world.
For these three sports, among others, the American league is the pinnacle of the sport. Players flock to these leagues from all over the world, just in the hope that they might one day make it. There is no convincing, no need to prise them away from other teams and dreams. America is the be all and all for these sports.
That is very much not the case for soccer.