Mostly because they're deliberately being made to leave the system so that it can be replaced by an worse American like system. For the reich to profit from.
Not only that, but the way that visa sponsorship and the citizenship process can absolutely fuck you over is absolutely horrifying.
I have a family friend who immigrated for an engineering role at a consulting firm. The job was going really well, and he wanted to stay in the US, so he was getting ready to start the citizenship process once he hit the 5 year mark. About 6 months before the 5 year mark, the company let him go, and he had to scramble within the 60 day grace period to find a new employer who was willing to keep sponsoring his visa, or else he'd have to leave the US.