Because of consistency and flow. And I can only imagine the technical headache it would cause trying to code something like that and make it work, by characters constantly stepping in and out, the camera angles and camera focuses switching, while keeping track of all the dialogue branches, choices and outcomes.
Companions already have the ability to interject during dialogue scenes with their commentaries, but if they wish to make them more dynamic and alive, then for such a thing it is better what UnknownEvil suggested. Having companion dialogue choices also present alongside ours and them interjecting if we select their dialogue choice.
My primary focus however is the main character, as the game does not respect it in terms of story and gameplay.
