Child porn is a unique case. Its presence normalizes child sex in the mind of its consumers, and feeds an addiction that greatly increases the likelihood of an actual action taken against a child.

It's the same logic most countries use to ban possession of firearms. Their presence greatly increases the risk of lethal violence and suicide, despite being harmless to people when used correctly at the range or for hunting. The lack of need for firearms weighed against the harm they cause to others makes it a hard sell for remaining legal.

Firearms may get a pass because they have real utility in self defense, but there are no positive benefits to having an endless stream of fake child abuse.