There aren't that many american talents to be kept in MLS
Your problems are two:
1. Soccer is a game for schools and colleges and not for the street. This game has to be learned on the street, when you're very young, 3-10 years old; you have to have mastered the basics by and only then you go and play organized football to learn the tactics. If you don't have the street/beach/footsal training, you typically lack the skills, improvisation and primal football instincts, needed to make it in the real game. No matter how physical the games becomes, athleticism is something that can come from outside, having skills and being able to read the game always come from inside, i.e, everybody can start running fast after going through the necessary training, nobody can start making Ronaldinho's no-look passes if he doesn't have it in him.
BTW, Western Europe has begun to experience exactly the same problem - fewer and fewer children play the game on the street and the consequences are already seen.
The same thing applies to basketball, but with opposite signs - the US players learn the game on the street and that's where their advantage in terms of skills comes from. I am sure nobody teaches windmill dunks and streetball dribbles in college or in high school, and basketball in Europe is a mostly indoor game
2. For some reason, americans simply don't understand the game. This is related to number two, but I think it also comes from the nature of baseball and american football, which are a lot more static and/or tactical games compared to football. I have played with people who play college soccer and who are supposedly good. All they do is mindless running around the pitch, playing rough, often dirty defense and even if they make some well organized play, it is always something standard. No improvisation, no tricks, no surprising moves, passes, no spark of a genius at all. If this does not change, they will never reach the level of Brazil and Argentina