Even if the imagery was 100 computer-generated without a training model based in abuse, at some point it becomes a line between “is somebody being hurt” and “what is acceptable in civilized society”. If we accept CG CSAM then what else? Gore porn? Snuff? Bestiality? How about a child being sexually assaulted and mutilated by animals at the same time? There is always stuff that’s going to push you envelope further and further, and how do you even tell if real individuals are involved, if it’s CG, or some combination of such?
If they’re all CG, then nobody real is being hurt BUT there’s still gotta be a line between acceptable and unacceptable. Society - at least western society- by and large has decided that the majority of those categories are not acceptable by law, so regardless of how it’s made it’s still illegal, and the dissemination of such can still have a harmful effect on society in general.