because stronger monsters is a such a bad thing.
There are only so many challenging fights* using 3.x combat:
-The enemy is weaker but more numerous
-The enemy is equal in strength and numbers
-The enemy is stronger but fewer in numbers
If the enemy is stronger and more numerous, you are in trouble. You can only push things so far with super-genius tactics, at a certain point math will take over and you will simply lose no matter what.
Fact is, most players will avoid such a scenario precisely because they know it's hopeless. They will try to even the odds, reduce the enemy numbers, or change the battlefield in some way to gain an advantage. Think outside the box.
But this is a CRPG which means you are only allowed to think outside the box
in ways the developers already thought of. In the case of the bandit fort there are a few different ways to even the odds, but I can think of dozens of options that players would use in such a PnP scenario that are simply impossible in the game. This is why I think balance is actually more important here than in a PnP game.
*(Some other examples would be a "puzzle fight" where the enemy is hard to fight unless you exploit a specific weakness, then it imhoecomes easy, or a gimmick battle where the enemy has some huge advantage but the players can nullify it or reverse it in some way using some device or aspect of the battlefield, like using a plot-artifact.)