A Wholly Unoriginal Idea

[Bonoki]

Would hurt their pride. That's my assumption.

It's the same reason they don't open the door to allowing modders submit content to the game through some sort of dev approval system in the same manner.

While I don't deny pride doesn't play a role, I think it's actually far more complex than that. There's a lot of ground to trend when it comes to things like art theft, copyrights, credibility, ownership, ect. They can't just take submission ideas willy nilly because if they used those ideas then people would want rewards and recognition and SE probably doesn't want to cross into that territory. That's why modders are treated like competition rather than comrades. Also artists are very wishy washy when comes to permissions.