Well, compare the number of characters and the number of quests to your average game. Not very reasonable to expect developers creating hundreds of hours of content with thousands of characters to be writing at the same level as an 8 hour action game. Providing good characterizations for a dozen important characters; piece of cake. Now do it for a few hundred. Branching narratives for a few dozen quests; piece of cake. Now do it for a few hundred. It's obvious that most of the characters in Skyrim lack depth and that the quests are shallow. It's also obvious to me why that happens to be the case. Skyrim is about quantity, not depth. BGS games have always been like this.
I think they could manage good characterization for a dozen important character though, right? They don't though. They don't really provide much characterization for ANYONE. Not even Ulfric, Tullius, Alduin, or any other main characters. All the characters lack depth and are shallow. Some more than others. They could do this for a handful of characters, yet they don't. Honestly, they could also bother with some decent AI for their NPCs since they have this big open world, but the AI for their NPCs is pretty dang pathetic.
Obviously it is about quantity and they don't bother having quality in the story anywhere. Yes, they've always been like this, which is why the writing has always been poor. It isn't that it is impossible, it is that they just don't care enough to even make a few characters have depth and a lot of characterization.
You're right, good stories don't require a background. They're just a lot easier to write if there is one. Hence 99% of the games on the market provide you with a character that the developers can tailor any way they like. The best narratives have always been written around strong protagonists. Even in videogames.
Good stories in games always have characters you feel strongly about. People naturally feel strongly about their own character. Job done there. All that it needs beyond that is making the NPCS have strong stories and grip people. And that's not really the case in Skyrim or any Bethesda game.
I mean heck, compare it to Dragon Age. Your background doesn't matter much at all in that game beyond the fact you're one of the last two wardens. Yet it still has a strong story and it doesn't have to rely on a lot of character background. Bethesda could do the same with the main storylines in their games easily enough. You don't need a strong and determined background for a main character in an RPG as long as you let the player interact with people. The player will provide the background, motivations for their character, etc, making the writing job a heck of a lot easier in that regard. A ton of characterization about the main character in an RPG can actually take the player out of the story if it is supposed to be a sandbox, but a ton of characterization for NPCs? That brings someone into the story and helps make them feel part of it. Especially if they have meaningful choices to make.
The odd thing about TES games is that while they are sandboxes, the stories aren't sandboxes at all, especially the main stories. They rarely have meaningful decisions to make. The stories are about as non-sandboxy as you can get. It's a weird dichotomy.