This isn't the case in law in many countries. Whether an image is illegal or not does not solely depend on the means of production; if the images are realistic, then they are often illegal.
Don't forget that pornographic images and videos featuring children may be used for grooming purposes, socializing children into the idea of sexual abuse. There's a legitimate social purpose in limiting their production.
It's very unlikely AI generated child porn would even be illegal. Drawn or photoshopped photos aren't so I don't think AI generated would be.