1. You are potentially giving a shield of deniability to people who create or distribute real CSAM because now they could claim that the images are just AI generated and therefore "harmless"

2. Efforts to stamp out real child abuse may be undermined by a flood of AI-generated false positive imagery

3. When people see something over and over again they start to think that it's normal. AI generation of this kind of material (something which can be done at a huge scale) risks normalizing the sexual abuse of children.

I'm sure there are many other arguments beyond these.