I don’t know what the situation is for commercial games — I don’t know if there’s a marketplace like that — but I do remember someone setting up some repository for free/Creative Commons assets a while back.
goes looking
It’s not highly-structured in the sense that someone can upload, say, a model in Format X and someone else can upload a patch against that model or something like that, though. Like, it’s not quite a “GitHub of assets”.
I’m sure that there must be some sort of commercial asset marketplace out there, probably a number, though I don’t know if it spans all game asset types or if it permits easily republishing modifications.