There's no doubt that some individual games are more mod-friendly than others. The reasons you mention all work in favour of the TES games.
My point is, modability is not down to the underlying engine, but more how you build your game on top of that - how you organise the assets, how you structure the data and files to make things accessible, a logical system for how files and records override each other, and so on. I don't think there's any reason to believe that, say, Cryengine or Unreal is any more or less capable of making a mod-friendly game.