I think there's a couple of things at play...
Firstly, is Larian building off of DOS2. Which I think is a primary reason why we have "Origin Characters" whom also happen to be companions thus there's this awkward writing where you can experience these "Origin" paths without playing that character. Though, there's that weird thing where in the opening cutscene, you see Lae'zel get infected from your point of view... Then you're asked "Who are you?" and you can then pick to BE Lae'zel which makes literally 0 sense.
Secondly, would be them trying to recreate the feeling of D&D. Where campaigns are not written with characters in mind so players can explore their own character building. Which works for TT gaming, but falls flat with a video game. This "Let the players make the character" type of writing probably is exacerbated by the core D&D system regarding backgrounds. It's hard to write for a specific character when players can opt for such varied backgrounds like one Tav might be an Urchin. Another Tav might be a Noble. etc. Even if the choice of background serves little importance to the actual story (There are no background related dialogues or choices, it's simply a means of deciding what 2 skills you gain proficiency in)
Personally, I've never really cared much for "Origin Characters" I'd rather create my own character than use a pre-made one. Even more so if said pre-made characters can instead be companions, allowing for all that writing to go into a companion that I can interact with throughout the story (As opposed to rando "Hireling" type companions that are soulless and boring)
I'd much prefer writing to focus on having your own created character fit into the story. Even if some options are taken away (Like backgrounds, name, even race) since a partially customized character that fit into the narrative always feels better than a fully customized character that gets shoehorned into a story while having no relevance to it.
