I'm sorry... No background to them? 'Cause every character in say, Fallout 3, had a background... Or maybe in Oblivion... Where the only background they had was worth a sentence in Wikipedia and the underlying paragraphs just detailed their schedule.
Backgroundtastic.
They all had schedules, they all had names, they all did things with their lives. Their AI is very complex (and taxing to the system to boot). AC citizens don't even exist outside of seeing them in the streets, and if you killed one their brother certainly wouldn't hold a grudge (like they will in Skyrim). Sure, it would be nice to have thousands of NPCs, but where would they all live? There aren't enough houses (or interiors) for the people of AC to live. For thousands of NPCs per city, you'd have to have
at least 250 houses per city, presuming the average family has 4 people in. BGS doesn't make games of that scale; they make small, detailed games. Maybe in 10 or 20 years and there's consoles that have the hardware to handle Bethesda's complex AI running on thousands of NPCs at a time, and maybe BGS will expand to 4 or 5 hundred members, so they have enough time in between games to make these massively detailed cities you speak of. Until then, I'm content with what Bethesda gives me, and would rather they start with less more detailed (and persistent) NPCs, and expand from there between games.
Have you ever tried making an interior?? It takes time, especially if you want to make the place feel unique. They just don't have enough time to make huge worlds with masses of houses with interiors.