Mods are actually an expensive venture on consoles as all content offered on their platforms needs to conform to the standards and requirements of the console itself. There are lots of ways that mods may not be acceptable for a console, whether it's something that could put the console into a position to become unstable or run out of available resources, or they include something that the console platform itself deems as inappropriate (whether nudity/sexuality, graphic levels of violence, or representation of political or cultural ideology.)
The extra layers implicitly mean that mods will have to go through a certification system to be approved. This is not only taxing on the studio and platform staff, but mods require updates whenever a game's code changes, like with any update. That means going through the process repeatedly over time, and that's taxing on the mod authors as well. If mods are not updated, they need to be removed or it would conflict with the requirements for providing products of certain standards and levels of functionality.
It can certainly be done, as Bethesda has proven with it's limited library of mods for Skyrim onward. This does however raise the question of monetizing mods to support all of this additional work, which adds a whole new bag of worms into the process.
So, while it's not impossible, there's a lot of consideration and commitment-over-time that would be required with the system as it exists today. Someday, perhaps...