That was never the argument. Wearing masks isn't about protecting yourself (only properly-fitted high-grade masks do that well, and regular people wearing those would have taken supply away from medical staff). Wearing masks is about protecting other people from you spreading the virus (which they do a decent job of when combined with distancing).
The reason it can't just be a personal choice is because not wearing a mask isn't endangering you, it's endangering other people. It's the same reason we don't let people smoke indoors. And having a blanket rule to always wear a mask in public was mainly about keeping the messaging simple. Constant debating over which situations were safe or not for going maskless would have harmed our overall responnse to the virus. The reason the CDC changed their guidance is that there's now enough evidence to confidently say that vaccinated people can't spread the virus to others, but honestly it may potentially cause those same issues.