Oops thanks for putting that out, corrected.

For the first point, it might be more of a patent thing than copyright, because you can patent improvements you come up with for someone else's invention.

Though another angle might be that game studios want to avoid encouraging a freelance game improvement market where people look to financially gain from swooping in and making improvements to their games. It might result in improvements they already planned to make but hadn't gotten to being blocked by patents and license demands. I don't agree that this is something that should be avoided, though I don't think current IP laws would make this a desirable system for anyone other than lawyers.

That's not to say that it's legally impossible to figure out how to navigate pulling in community changes to the main game, there's just complications involved that so far Bethesda has preferred to avoid. They might even just want to avoid a case going to court to set some kind of precedent because it might involve paying royalties to modders. IMO they would deserve to be paid if their work gets pulled into the game directly or indirectly, and even just as modders adding value to the base game I think maybe they deserve some compensation for their efforts.