> If foreign researchers are able to prove themselves abroad and contribute as you say, I assume they can continue doing that in most cases without coming to the US to take jobs.
They can't prove themselves abroad. Consider the example of "Attention is all you need" paper. That paper came into existence thanks to talented people from around the world coming together in one place and then collaborating. It would not have happened if the contributors stayed in their own countries.
> If immigrants are such a great resource, why are poor countries not trying harder to keep those people from leaving?
They are. See here: https://www.reuters.com/world/asia-pacific/china-restricts-o...
> Why doesn't China import a bunch of Indians for IT work, or vice versa?
They are trying. See here: https://www.nytimes.com/2025/10/14/world/asia/china-stem-vis...