We cannot deny that there is a brain drain in many third world countries that desperately need well trained workers that is accelerated by acceptance of immigrants into developed nations like the US. Who’s going to lead these countries into better governance and economic improvement if all the talent is incentivized to leave their home countries for greener pastures?
In contrast, it would be a terrible thing to forcibly lock people into their birth countries to stop brain drain.
For example, many third-world countries had tax-free regimes for tech/services export in order to boost these industries. This made sense to them. If you made $100K contracting for a US company, you'd pay $0. But you'd also get to spend/transfer that much needed money into the economy. You'd probably also not use any of the government services because they are sub-par to what you can get privately.
Now, if you are required to pay an income tax at par to what you pay in developed countries, the whole scheme will no longer make sense. No one will agree to pay $50k and get no services in return. So you end moving to some other destination (ie: Dubai, or a western country where you pay $50k but relatively get your money's worth).
This is as close to war as it can get.
This isn't hypothetical, of course. This is how places like the Soviet Union and East Germany worked. It was policy to literally shoot people trying to leave the country without permission. On the other hand, while certainly not justifying this atrocity, there certainly has been a "brain drain" as engineers, physicians, and scientists from the former Eastern Bloc have moved West for better employment opportunities.
What portion of this is non-voluntary?