My first instinct is that it's because everything can still be broken down into bespoke parts. You're not hooking up to the Copyright Infringement Pipe as a substitute for making content, you just made a thing that rearranges the content you've made. People can feel when something had a human hand creating it, and that's one of those intangible things you don't notice until it's gone.

I've seen procgen used for stuff like character icons, item/enemy behavior, and weirder shit like Dwarf Fortress's engraving descriptions. Never seen any complaints about it being "wrong" to do that, just "it looks bad sometimes"