Many of the best research universities in the world are in the US. 95%+ of the world's population is not. The best schools have access to the global talent pool

Many of the best research universities in the world are in the US. 95%+ of the world's population is not. The best schools have access to the global talent pool, so if the system is working even fairly well to identify great students, you would expect most PhD candidates to be from elsewhere.