It's possibly that the key U.S sports aren't as popular outside their borders as inside (so if you want to make it in a U.S sport then you need to move to the U.S). As / if this changes there will be a bigger representation of the rest of the world in U.S sports and were things like basketball and American football to be embraced in a similar way to say football, then you may well see Amercian's heading elsewhere.
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