Real life =/= great gameplay.
Yes, in reality one or two hits tend to decide most fights, but if PoE played that way the variance of every battle would be huge, and the outcome would depends less on player skill than on random chance. All that would accomplish is a lot of reloading autosaves and hoping for better dice rolls.
Action games like dark souls can employ very lethal attacks because player skill can mitigate against damage, but for a 3rd person isometric RPG it would not be much fun IMO.