Bethesda is owned by Zenimax, and an officially licensed mod scene is completely different.
If you want to run the mods for Bethesda’s games, you need the retail software to do so.
I guarantee that if a group was creating a Morrowind remake that didn’t require owning some Bethesda core game that was being modded to achieve that, Zenimax’s lawyers would be quick to be on top of the issue.
It’s not like there’s not examples where Bethesda’s lawyers caused mods to be shut down where it involved redistribution of Bethesda game assets without needing to buy the game.