Academic salaries in the US are much higher than anywhere else. That's where every academic wants to work. Consider this, many places in Europe pay their junior faculty as little as PhD candidates in the US.
Academic salaries are higher in the US than in most other countries. But they are also unusually low relative to the cost of living and industry salaries. In many meaningful ways (such as home ownership), American academics enjoy a lower standard of living than their colleagues in Europe.
Moving to another country is a huge sacrifice and involves significant risks. Most people, including most academics, are not particularly motivated by high incomes. What they seek instead is stability. And that used to be the main reason why American universities were able to lure PhDs from other wealthy countries.
Because academic salaries are unusually low in the US relative to the alternatives, there is less competition for academic jobs and grants. If you take the subset of academics who are willing to move to another country, they are more likely to find a permanent position in the US than elsewhere. Less competition means that any particular candidate is more likely to be the top one for any particular position.
