Because it creates an appetite for that type of content which is expected to grow to include real images with real harm.
Considering how many people want to murder people (or justify murdering people) simply for being attracted to children, are you categorically sure about that?
The only reason I personally can't condone photorealistic AI images is because they're indistinguishable from photographs.
And in this case, it's required the exploitation of another human being (their photograph, or a photograph of them) in order to undress/manipulate the image.