This is the main point, it is about the type of training. My exercise physiology class went into a lot of detail about this. Different exercises affect different types of muscles. More aerobic/endurance training is going to more strongly affect slow-twitch muscle fiber. Anaerobic/resistance training affects the fast-twitch muscle fibers more. Around the world, soccer academies are the norm, so the kids are going to be going through more endurance training for most of their lives. In America, football, basketball, and baseball require short bursts, so resistance training is much more common. That is why having Americans in soccer training for all of their lives isn't going to produce the typical American athletes that play basketball and baseball. The key would be sway athletes who have trained for the other sports to choose soccer and not the other sports. There are always going to be exceptions like Thierry Henry. However, because of the type of training they go through, soccer players throughout the world will rarely be able to match the speed of the fastest players in the NFL.