This has been bothering me just recently, I remember playing Morrowind and vastly modding it to improve on points like character detail and the combat system, even adding a hunger + thirst mod and a drivable boat.
Easy stuff, took no effort to get working.
Now I decide, I want that stuff again, and this time for Oblivion. I have a boat mod, increased character detail, scythes, more bows, this and that and the other. Now the Readmes make it sound simple enough, extract to the data folder, just like Morrowind. Okay, shouldn't be a problem. I do that and turn them on in the data files menu and when I get on Oblivion, there's zero difference.