As someone working and doing code interviews for a large company that pays well (even by bay area standards), I think it's a lack of qualified US citizens. The number of graduates I've interviewed that can't figure out a basic graph traversal is crazy. I don't ask (or expect) them to recite algorithms from memory but even if they haven't worked with graphs before, they should be able to figure out a BFS or DFS on their own and suggest some ways to improve on it. Many of them however, cannot.

I'll also note that I don't interview many foreigners and the reason I see mostly new grads is that I'm not senior myself, so it makes little sense for me to interview senior candidates.

At the high end, there's simply a larger supply overseas. The US has a population of 300M and between them, India and China have about 2.7B people and their countries pay far, far less.