I agree with the OP that dual wield falls behind 2h, but I think that this is a feature, not a bug. I phrase it this way, because I actually like the way that 5E sets up the distinction between the two styles.
If you are strong and want to do really beefy damage, use a 2h weapon (you will probably need heavy armor for defense in this case because it is difficult to invest in both strength and dexterity).
If you are not particularly strong, dual wield gives you an option to partially close the gap, at the expense of your bonus action. If you invest in dexterity you can achieve decent defense without heavy armor in this case (need 15 str to equip full plate).
If you belong to a class without a 2 attack action (for instance, rogue) then dual wield gives an extra attack that you would not otherwise have. This is particularly useful for rogues as it gives an opportunity to land a sneak attack if your main hand misses (of course Larian has not implemented this properly, but *hopefully* they will).
If you belong to a class that has riders on the attack (ranger with hunters mark, warlock with hex, or anybody with a flaming/poison weapon) then adding that extra off hand attack might be enough to come very close to 2h with GWM.
In summary, dual wielding IS behind 2h in raw damage, but this is as it should be.
