Maybe they still use it, because it serves their needs so well. It's just my feelings, but looking back at Oblivion, Fallout 3, Skyrim and now what I know about F4, they just seem to update the visuals. Nothing else. Not any groundbraking changes or improvements. And with the visuals, just enough that it isn't completely outdated. What their games do offer though, almost always, is an ridiculous amount of items to interact with, from spoons, to herbs, garden gnomes, teddy bears, and totally ridiculous junk, that has absolutely zero meaning for the game or gameplay. You name it! Seriously, I never saw a game that featured so many useless stuff that you could pick up ... for ... no reason?
They say it is immersive. But I feel, it just drags their engine down, like a lot. All those cells in the world that have to keep up with the changes and interaction with the player and items. Every dungeon you visited, every stuff you collect and throw in your home(s). It can get to quite a lot of informations I guess. Maybe that is also the reason why they have always so many issues with their game and so many of the same bugs. But all of that is just a guess of course.