It's because it's more like a chess match than a final battle.
You've manouvred him into a last desperate act, he's overly weak, and you get to cry "Checkmate!" and knock him over.
It took me a while to realize that the true final battle was the Temple of Mythal.
Was it? Well that wasn't much of a final battle either. I sided with the Elves at that point.
The way I think it should have ended was Skyhold gets attacked, perhaps you are on your balcony when the dragon comes and shoots a fireball through your tower. (Gives it a reason for being there) Depending on whether you have completed Companions quests, they either live or die. You repel the attack, and decide to retaliate. You go to Haven, where there is another attack. You choose your party whilst the rest of your team and the main bulk of your forces that have returned from Arbor Wilds stay to fight Corypheus' army. Then the final battle begins.
I get that that is sort of like ME2's ending but that was a good way to end the game.