[donatj]

It's the same as making a remix of a song. The base of your new version is still the original authors work, so the author deserves a cut. A mod is akin to a remix of the game, and that remix is still mostly Bethesda assets, not to mention running atop their engine.

[yincrash]

The game still has to be paid for to play the mod. Bethesda _already_ got their cut when the user paid for the game before even considering the mod.