Bethesda + Piranha Bytes make the world. They each have strong points. Then give that world to Bioware who writes the main story and puts that in to the world. Finally, Obsidian does a lot of side quests and populating the world. They were really good in New Vegas.
I'm really not sure who I would want doing the combat, mechanics, and what-not.
Sadly even though all 4 companies are great they each individually fail at certain things and excel at others. If only they could each play to their strengths in the same game.