Morkus from Orkus
I think the more likely answer is simplicity. They could have done like 3e did and had a rule for when you both can't see each other. Another rule for when you can see the target, but the target can't see you. Yet another rule for when you can see the target, the target can't see you, and you are behind the target. A further rule rule for when you can see the target, the target can't see you, you are behind the target and you have a duck on your head. They didn't, though, they went for a much simpler, leaner rule set.Yeah, or everyone is so good that they've trained a least a little in blind-fighting.
They decided that if you have the advantage, you get 2 rolls and pick the highest, whether it's a small advantage or a major one. Then, because people might not like all advantages being equal, they went on to tell the DM that the rules serve him, not the other way around, that the rules can't cover everything, and that it's a rulings over rules edition. That way if the DM thinks that having a duck on your head makes things super awesome, he can just say, don't bother rolling, you hit.