Excuse me. The USA have a ridiculous system of world wide taxation (like Eritrea). Sufficiently qualified people might want to work there for a while, but not become citizens.
If you force the green card or even citizenship onto H1B employees, they might not want to come.
Furthermore, if you're on a green card (or a citizen), you're subject to the exit tax. On an H1B, you can avoid it for a few years at least.