> Why would a person who is supposedly having rare skills come to a country and spend their best years here without reasonable guarantee of permanency?

For the same reason I temporarily resided elsewhere: An opportunity to expand my horizons, experience another culture, and oh yeah... the money. In fact, through much of history it was common for skilled workers to go abroad and bring skills home, this was how industry got kickstarted in many places and there were even various formal programs for this. This is even the case for skilled labor that doesn't qualify for an H1-B visa. It's commonplace for aspiring chefs to work in kitchens around the world under the tutelage of well-regarded chefs to learn from them and then apply the same techniques and operating models to their own restaurant at home.

You are mentally applying reasoning that assumes permanency to a situation that explicitly says it doesn't include permanency.