Because they don’t make money from mods and this was an obvious cash grab from the beginning. If Bethesda had any good ideas they’d be making a game out of them. They don’t, so instead they’re reselling you the same game they already sold you 19 years ago with a fresh coat of paint on it. A million people will buy it anyway because nostalgia, Bethesda gets their money, and whatever happens afterward is not their concern.