Agreed with this. It's a mechanical issue, not a thematic one. They could add 30 pages of lore to dialogs and random NPCs and it would be a snoozefest because "lore" isn't actually what anyone wants. The Last Days or Warsword Conquest were impactful not because of the lore of their source material but because of all the additional systems and mechanics added in that back up that lore. As Bannerlord is, if you changed the names and icons, you would have no idea who any of the factions are - the theme is only surface deep, the same as Warband was, because making a bunch of distinct mechanics for each faction or clan is a ton of work and just making stuff that applies to the entire sandbox is more efficient use of your development time.
For example, consider the Empire-s. As-is, they're identical, just different shades of purple. Instead, what if the Northern Empire had a Senate system, where the Emperor will be deposed if they fail to keep the land secure and prosperous, and so the North tends towards peace and trade and the ruler has little influence over policies, being voted in and out every so often by the Senate. The Western Empire is ruled by its military, and the ruler's influence is a direct function of their military achievements, at risk of being replaced by another who has achieved greater victories or avoided embarrassing defeats. And the Southern Empire, remaining a hereditary monarchy, is ruled with absolute control and the rulership remains tied to the imperial bloodline, never leaving the Pethros clan.
Don't think too much about the balance or gameplay or historicity or whatever of those ideas, they're just demonstrative. The Empires would be more distinctive, and with each additional layer of uniqueness like that, each faction and clan and character in Bannerlord would feel more fleshed out and interesting. When people complain about the lore being bland, that's what they want.
(And I get why it's not there; it's a lot of work and it's all non-transferable, those weeks spent on the Empire do nothing for everyone else compared to the same time spent on, like, trade and battle mechanics, or whatever. Theoretically it's something modders could take a swing at, but the stumbling blocks for modders are well known at this point. But that's what people ITT are actually asking for.)