You badly mistake what the Creation Kit is. It is an internal development tool used for making the game that Bethesda kindly lets us use.
They don't spend a penny developing it for the user base. Any bugs in it when they throw it over the wall - stay bugs forever. It is not maintained.
Any costs associated with the Creation Kit are development costs associated with making the game.
Mods also have not cost Bethesda a penny in lost revenue. They drive interest in the game, and help develop a loyal community.