GC holders are permanent resident aliens, not naturalized citizens. Yes, thet went through a legal process, but citizenship is further down the road of that process.
Which you can do 3 years after marriage (in the case of Khalifa), and it’s recommended to do so.
The main reason people don’t is because they don’t want to be filing U.S. tax returns the rest of their life if they plan to eventually move back to another country. I have relatives who have made such a decision.
