For drawings it has to additionally be "obscene" (since obscenity isn't protected by the first amendment). And there is also a specific law that criminalizes even non-obscene realistic computer generated imagery.
The reason why the Supreme Court upheld bans on possessing CSAM is not because it's obscene, but because it incentivizes abuse of children to produce it.
We also acknowledge that participating in pornography requires consent, and that the continued distribution of nonconsensual pornography constitutes a continuing crime against the unwilling subject. Because children have zero legal capacity to consent, CSAM is de facto illegal.
It misses the point to think that CSAM is illegal because it is "obscene". It isn't illegal because it's disgusting; it's illegal because it's egregiously harmful to children. It's like thinking the bad thing a murderer did was make a mess.
Are you sure you're not the one missing the point here? You're replying to the tail end of branch of discussion about AI-generated imagery. Obscene is the only argument that works against it.
Nonconsensual pornography is nonconsensual pornography. If a real child is involved, it is criminal.
With that said, purely AI-generated CSAM, while certainly disgusting, does pass the test for free speech, in that there are no victims and it is purely offensive, not actually damaging.