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.