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.

Correct. One has to file an N-400 to get naturalized after obtaining LPR ("green card") status.

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.

Nope. Not at all. They put people on a path to citizenship, but they are not yet, till they are sworn in as a citizen. Residency can be revoked for criminal behavior, fraud, among other things.

This is not correct. But I can't edit it :/ so I am just adding this comment here

> GC holders are naturalised citizens

Incorrect. GC holders are not "naturalized US citizens" and still need to apply for full US citizenship with the USCIS when they are eligible to do so.