I think you're making assumptions that aren't fair but maybe aren't obvious either. Honestly I'm only thinking about this because I just watched the contrapoints video on twilight, and so I'll use her example, though she's talking about a slightly different topic. Gonna paraphrase like a mofo:

Weird Power dynamics between partners in a fantasy, like twilight, or say porn since we are being obvious here, is normal because self image often requires women to present one way while hiding their desires for sex. It's absolution of a sort, and is ostensibly healthy to explore in this way. That said... Some examples such as race play in fantasies may dehumanize the "other" in super not cool ways and reinforce negative racial stereotypes.

If we take that and extend it to pedophiles, normalization of the thought process leading to that sort of disfunction/disorder seems like a not great thing, but yeah, we'd need to study to learn more and that seems both difficult and likely undesirable for the researchers.

Why would it be any different?

Because pedophiles display pathological deviation when it comes to sexual attraction.

Because "CSAM" states abuse as the third word in the acronym. Machine learning could (in theory, I lack knowledge on the current implementations) be trained without any children being abused (in any traditional sense anyway) and used to produce the content without any real children being involved (ignoring training data).

The downvotes likely come from a difference in definition between abuse and CP, images of nonexistent people cannot realistically harm anyone.