It's been 20 years. Standards have changed. A Lot.
Each new companion is a custom model, a writer, new animations, and a voice actor at minimum. If each companion has their own companion quest, then it is designers, more writers, more animators and more actors, and much, much more time and money. We are just not in an era anymore where you can bang out generic companions in an after new with a junior staff writer and a portrait.
Sad but true: Once you go full voice and performance capture, you have limits... if Larian needed to do that for a Baldur's Gate game? Well, that's a different question. Personally, I wouldn't have minded reading, and reused animations are usually fine if they are subtle enough.
