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.
You do know that BG1, which was developed in the 90s, had over 34 voice actors (as officially listed in ImDB) for inumerous character and throughout dialogues in all game, they had an extensive amount of characters with different personalities and really unique, yeah Bioware rocked the narratives in the 90s and early 2000s. It's actually just a matter of time and effort instead of being an impossibility. It doesn't look like an issue to me to add more unique companions for one of each class at least after all there are only 7 more potentially missing from the rooster.
Sure animations and facial capturing might be a bit harder but they own that technology and it isn't a matter of recreating the wheel, just using it. Might be time consuming but that's why they opted to release it as EA instead of waiting 4 years to release a full complete game.
