The majority of countries people would actually want to work in. All of the EU, UK, countries like Australia give permanent residence to Global Talents (I've recently got this visa), also accepted by NZ. It's much easier to get a work permit for countries like Singapore, Hong Kong, Korea, Taiwan (no silly several months wait/October-only start date/raffle for H-1B) etc.

I love the US, lived there for a decade and would love to go back to California. But I'd never go on H-1B and I don't see anyone sponsoring me a green card again (returned mine seven years ago). So there's that.

TL;DR: The US is not easy to (legally) immigrate to compared to many other places.