I was on the (now-closed) Insomniac Games official forums for a while, where some of the game devs were quite active - and they basically said they can't accept fan submissions outside of official, targeted competitions ("design a costume", "design a weapon") because it can be a legal issue - I think to do with credit or compensation or something like that.

It would also be difficult for their own original work. Imagine if a fan submits something coincidentally close to what the game team were planning to implement, but haven't publicly shown off yet. The fan might then accuse them of plagiarism or taking their idea and not crediting them for it.