Its not: you get to directly address a shortage in your country without the burden of raising and training the person. Arguably, if you don’t mind morality, the immigrants are also easier to exploit. And all that + you get a free tax payer!
My reading is that Americans (but also other parts of the world) are 1) uneducated on this topic 2) racist and or xenophobic to the point of self sabotage.
Quite literally if county building was a video game where the xenophobia of the masses could be ignored, I would brain drain the shit out of every country, leaving myself stronger and the rest weaker.