Yeah I'm sure Microsoft-owned Bethesda is shaking in their boots about learning from modifications to their own game. That's gotta be everything stays buggy.
If an employee writes code for a company, the employer* owns the copyright.
If an individual writes code on their own time, they own the copyright.
If someone publishes a free mod containing code, that mod could contain a combination of that person's code, code from other contributors, and even other copyrighted code that none of them had the right to in the first place but it either hasn't been noticed or isn't being pursued because there's not likely any money in it anyways.
It's that murky area that I'm guessing they'd want to avoid. They might be more likely to hire the modder to do that again from scratch for them than to use their work directly. Blizzard did that back in the day with two (that I know of) of the people writing modding tools for StarCraft. Their tools remained on the modding site and were never officially adopted by Blizzard but the authors worked on the WC3 map editor to add some of that functionality right into the official map editor that was going to be released with the game.
Edit: corrected a mistake where I said the opposite of what I intended to (that the employee owned the copyright rather than the employer)
Hiring the modder is not necessary, to look at a mod, go 'oh that's what we did wrong,' and fix it. That's not the ctrl+c/ctrl+v situation you seem to expect. And considering it's their own game, and fixing bugs, the legal concerns are practically nonexistent.
If an employee writes code for a company, that employee owns the copyright.
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.
Just generally rambling about reasons why companies might not want to adopt user-authored changes in their main game.
There's copyright that applies to code (which would cover copy/paste). There's parents that apply to ideas (which might still cover cases where you didn't use copy/paste). And there's precedence where if you do something one way one time, others might expect you to continue doing it that way even if you intended it to be a one-off (which might overlap with both of those).