I’d say it’s because the person you’re replying to rightfully sees it as a slippery slope. If you say this fake image that didn’t directly harm anyone is illegal, what’s to stop you from saying some other fake image that’s much more in line with social tastes is also illegal? Ie an artwork made of human shit, for example. Most people would be repulsed by that. But it doesn’t change the fact that it could be art. As long as it doesn’t concretely harm someone, it’s hard to equate it to said harm.