And a lot don’t leave if their spouse can’t work in the US (or has no interest in being sah). I know lots with partners who would never give up their Canadian gov pension track. That and other family obligations make it harder to move.
Many do similar calculations. Home is home. It's profoundly arrogant (and very American) to assume that the "best talent" moves for money and opportunity.
Americans have very high labour mobility rates, and often assume that's how the whole world works. But in most places in the world the idea of uprooting your whole life for a job is seen as odd.