The problem is not that the companions are fleshed out. The problem is that Origin Characters have backstories and tadpoles whereas Tav only has the tadpole. Thus, the only thing that makes Tav unique and actually feel connected/important to the world ([Baldurian] dialogue tags are not sufficient) is not, in fact, unique.
There have been some complaints that seem to revolve around the player not feeling like the most important person in the game because the companions have proper backstories. I am not sure how a premade backstory will help those types though, they probably want no companions and only followers. For the rest of us, we have the freedom to be as unique as we want in creating our backstories.
There is a big difference between being the "most important person in the game" and "having a unique in-game connection to the world (not just headcannon in your mind)." Currently, Tav's only in-game connections to the world are the tadpole and the [Baldurian] tag. See @Tarlonniel's comments above on ending up disconnected from the story. I'm not necessarily asking for a premade backstory. Tav's connection to the world could also determined by a more detailed tagging system: e.g., personality traits tags (honest, kind, lawful, violent, etc) and more locations for our character's hometown than [Baldurian] Or Tav could have a more unique connection/relation to The Absolute, played or narrated through right before being abducted
I disagree with your assertion that "[players who want Tav to be the most important in the game] probably want no companions and only followers." Afaik, the difference between origin characters and followers is only the ability to start the game with them & the tadpole. Backstory freedom can be almost entirely separated from NPC stories/importance/strength of personality.