> one can be convicted of CSAM-related crimes related to paintings/drawings/created_art of fictional people.

This isn't necessarily the case in the US, though I believe only for drawings. AI-generated CSAM probably wouldn't fly in a court of law.

Regardless, it's a naive conception of a system of law to think of it as a utilitarian system of restitution in contexts of "this individual harmed this individual". In fact, that would fall under the category of a "tort" rather than a "crime". The law is just as much about enforcing social mores and norms as it is about dealing with individuals harming each other. Hence why locales like Canada outlaw all forms CSAM, even fictional ones. The victim taken is to be society itself. The possession of this material, implicitly entailing enjoyment of it, is so gross a violation of society's norms and mores that it becomes elevated to a legal matter.

> The law is just as much about enforcing social mores and norms

This shouldn't be the case in a society that supposedly values liberty.

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.