Larian Studios Forums Divinity: Original Sin 2 Divinity - Original Sin 2 - General Single Player, Multiple Characters

Part of the design is that the characters are supposed to be able to keep secrets from each other, and a lot of the design is focused on the idea that you'll surprise each other with your betrayals.

From seeing this, it's easy to see that the point of competitive questing is that it provides narrative motivations that help to foster rivalry between real players. Co-op is the entire point.

While it technically can work in single player, the experience they are trying to create is NOT a single player experience and they don't see it as improving the single player experience.