I'm not an ATC, but I think there's a clear need for awareness of potentially conflicting traffic. If you divide that traffic over more people, you need to add communication between the controllers in a way that you don't when it's all handled by a single person.

That's not to say there's not ways to divide it up, but it's not always easily divisible. Well implemented technology can help, but poorly implemented technology can hurt, so everything needs to be done slowly and carefully.