Consider the fact that we get to name our character, and that almost all non-voiced scenes refer to you by your character's name. If they voiced ALL dialogue, they would never be able to refer to you by name but would have to say she/her/WoL/"hey you" which would get extremely tedious to write around. Not only that, but it would also dampen a lot of the relationships we build with the npcs, like if one of the twins say "thank you for saving me there, John" it acknowledges you as a person, whereas if they said "thank you for saving me there, Warrior of Light" it comes across as much more formal and distant.