Probably this, but despite that people keep trying - e.g. Reddit's gaming forums are full of "I made a concept for xyz!".

I mean it can work; especially for smaller studios, community members and modders are often hired to work on the game itself (I'm sure Bethesda has a lot of that, the modding community is basically free onboarding / training, but also Factorio's Space Age was mainly inspired and executed by the developer of the Space Exploration mod).