*Facepalm.
You are wrong, Bethesda simply cannot optimise for [censored]. Crysis 1 was pretty much open worlded and it's maps were HUGE . Plus the gpu and cpu are seperate things that do seperate tasks. The only thing that brings forth the misconception that "open worlded games cant look as good as linear ones" is That often open worlded games need to put quantity into their art assets rather than quality.
Yup, Crysis maps can be huge.
Also, Bethesda's engine is a pretty big joke when it comes to distant objects, which is basically the whole purpose of this "new" engine for this open world.
The distant LOD is a joke: http://static.skyrimnexus.com/downloads/images/1598-1-1323264331.jpg
Looks exactly like Oblivion 6 years ago, but it got a few more distant buildings here and there. Not only this, but the game doesn't even stream distant lod objects properly. They pop up in huge quantities at a short distance, again a joke.
So when saying that Skyrim's open world is because of Skyrim's not-as-great-graphics-as-other-games, that's [censored].
I think it comes down to 3 things:
1) unoptimized engine as hell. TESVAL and SkyBoost has proved that.
2) Bethesda decided to spend very little time on tech, because they prioritized other things instead, like 300 hours of gameplay.
3) All the scripts for quests, AI for so many NPCs, Radiant Story, etc etc is one of the reasons due to low performance (but I may be wrong).