Many people have no idea how these things work.
For example, right now, as a Brazilian, I can't accept invitations to speak at conferences in Europe because my visa to work in the U.S. has expired.
I am not illegal; I have a work contract with the University of California until 2027. Still, I must travel outside the U.S. and get a visa stamp on my passport to allow me to move freely to talk about science.
I have to do this every single year I stay in the U.S. The costs of the visa, traveling, time lost, etc, are all on me. Not every international scientist can handle this.
This is a reminder that international scientists face various hurdles and citizenship in some countries is worse than others. I had 2 students miss 4-6 weeks this year JUST to renew their visas (India and China).
(have to admit that global south was confusing, coming from NZ)