this is possibly because generated CSAM may look like no real human child nor resemble any human child in particular today. give it 5 years and physical distinction from real children and even a specific child is likely to happen. so a photo perfect likeness of your kid ends up in some disgusting video and the creators get off the hook because it's 'not real' by say a narrow color gradiation or kinematic similarity standard undetectable to human eyes? no thanks.