Justice Thomas too forecasted that "technology may evolve to the point where it becomes impossible to enforce actual child pornography laws because the Government cannot prove that certain pornographic images are of real children." In his view, "if technological advances thwart prosecution of 'unlawful speech,' the Government may well have a compelling interest in barring or otherwise regulating some narrow category of 'lawful speech' in order to enforce effectively laws against pornography made through the abuse of real children."
Every law is, fundamentally, a restriction on someone's freedom. The key justification for why that's okay is that the law protects one or more people from harm in the process. So in this case the question is "who is being harmed when an AI generates this particular configuration of pixels? How are they being harmed?"
When you dig into that it's more complicated than the simple knee-jerk "that stuff is just plain bad!" Attitude that has been drummed into the general public.