I fully agree that not being able to have backgrounds impact dialogue is incredibly limiting and kind of disappointing. I feel as though backgrounds give so much extra vibrancy to a character, and when your companions are also essentially pre-made characters who have deep, pre-set backstories already implemented, I think that a custom main character should be at least as interesting as them. I mean you can play as a vampire spawn with a uniquely crafted backstory that impacts the current events of the game, or a member of a race that has a personal beef with the Mindflayers, adding an immediate extra dimension to the conflict. When you have options like that, then from a narrative perspective what's the incentive to create your own character whose background is basically incedental or even irrelevant to the story at hand? Sure there's the mechanical aspect of wanting to play around with builds and what have you, but in a game clearly so focused on narrative and story, I don't think that's an adequate counterargument.
In the original pillars of Eternity game you pick a background for yourself. Not only does it get referenced at times throughout the game and open up extra dialogue for you (not a lot but still some) but you also have a conversation right near the beginning where you talk to someone and get to expand on your history, why you're out where you are and what you're hoping to achieve. Those questions don't have any in-game effect but they immediately went a long way-for me at least-in helping me connect to my character and build up who she was in my mind. I feel like that's what backgrounds can do in an RPG. Stats and stuff decide who your character is in the moment, but backgrounds let you connect with your characters past and shapes them on a deeper level.
