Going by your post, the video game industry still does a terrible job at telling the public just how difficult, time consuming, and complicated developing a big game can be.
I blame publishers for this.
Rockstar made Red Dead Redemption 2 and it looks amazing compared to Skyrim. Bethesda has made a lot of money from Skyrim, so I don't think they have financial difficulties.
But they have chosen to stick with, from my understanding, a heavily modified version of the Gamebryo engine.
Maybe it's taking a long time because perhaps they are building a new engine from scratch.