[...] 1 big-ish quest for each + one main cinematic at the culminating moment of the quest rather than fewer characters and A LOT of cinematics for each.
That's part of determining the right balance between "adequately developing a character", and "total development time invested in that character". You can spend a lot of time working on a character without that character gaining any more depth (like making more cinematics or recording more audio). IMO it is the "original characters which you can also play as main characters" idea that keeps them from simply churning out more companions. This idea means each of these characters has to have such a lengthy, complex quest that goes along with the main plot, when you have only so many maps in the whole game. "Oh, somehow all of my companions have the next milestone in their story happen in the same city/region/area that we're heading to, and the next critical NPCs we need to see happen to live in the same neighborhood. What a coincidence."
"We make our choices and take what comes and the rest is void."
