Couldn't agree more. There is certainly a lot of room to improve TES' storyline, emotional depth and writing in general (outside of the Lore). It is sadly something that carries over to FO as well, and that becomes crystal clear when another developer (Obsidian) takes over the franchise and ends up making a better story with a miles better writing team (Vegas), though I'm hoping this trend is about to change with FO4.
But when we're talking about TES specifically, I think it is somehow a deliberate thing, they don't want to go into too much details with the Main Quests because it could potentially hinder Bethesda's plans for future installments, and we all know how evasive they are about previous games' major events in each new entry.
That is also why the side quests are usually better than the main quests, it is a much more local thing, so it gives the developers a lot more room to work in.
I think that if I came across this thread on 5 separate occasions, I would give 5 different answers.
On this particular occasion, I am going to go with the immersion. I play a lot of fantasy games and a lot of RPG games. I read a lot of fantasy books, epic especially, and I love it all. No game or book has ever made me feel so immersed in the world as I do with Elder Scrolls (Skyrim in particular). They don't give me the same feeling of ownership of the character and his or her story.
I think that it is part of why I am so hesitant with TES Online, because it isn't 'my' world.