There is simply no need for it. The game is amost perfectly balanced to your own level if you follow the "right" path. You will always fight against mobs that are on the same level as you or one level different at the maximum...
There's no "need" for any of the things in the game; their purpose is to be entertaining.
True. But there is something called "design" and "balance". By adding respawning enemies you would cripple any serious attempt to properly balance the game.
If you just want to enjoy combat there should be combat-only maps soon. But in the campaign you follow a certain path which is well balanced for your party to offer constant fair and balanced challenge.
