You're guessing wrong, I've made a great deal of mods for personal use for both Oblivion and Fallout 3. But by all means please explain how you can create a new campaign in the originals without an ungodly amount of scripting and tinkering and also having it be completely separate from the OC. I need a good laugh.
Simple. YOU CAN'T! You still have to build every quest, every faction, every item and character and spell and script. How in the hell do you expect the CS to just make a new campaign at the click of a button. What is this Age of Empires? You can make a complete overhaul yourself. But to think that the game is going to just build the quests for you is imbecilic at best. What else do you want it to do hire voice actors, make meshes, how about it makes a cup of coffee while it does all the work for you? Madre de Cristo...
Edit: Nehrim was made in the CS. You think someone else has a program that can make Oblivion mods besides the CS? A lot of the models and textures were hand done but you had to use nifskope or whatever to do that anyways. All you need to do is make a new worldspace and new cells for your new area. It really isn't hard, just time consuming.