What are the reasons a company would not hire remote outside their own country? Limiting to only one country doesn't kinda defeat the idea of hiring remotely?
Aside from timezones which has been mentioned. I think handling taxes with foreign employees is more complicated.
Those companies may feel they have access to enough talent in the USA. They may have restrictions with data protection or security. They may be worried about timezones or communication. Or they may be worried about complying with foreign employment and tax regulations.
I'm guessing it's mainly about time zone management - working remotely in different timezones is a lot more difficult when they vary significantly. There's also the issue of "right to work" in your country to consider.
I would imagine that, but I see many US based companies that do not hire in Latin America. And Latin America has lots of good developers, so it got me thinking.