Couple of things make it obvious.
You don't develop things in Software Engineering with just a single team assigned to everything (obviously different on small projects), it isn't practical and it is unmanageable (one project manager across x people? No.).
Secondly, it's a game, people have skills in different areas, you can't just get a coder and reassign him to making dungeon fights or lore. Different skill sets are used in different teams to achieve different goals.
That's why BioWare always talk about "Have to speak to the art department" or "Have to speak to the writers".
It's a segmented effort. Standard business and it should be common sense for most people.