This adds to a living world that you have an impact on through your quests, and not just through slaughtering mobs whole-sale.
Original Sin is neither Skyrim nor an MMO and it's also not a real time action game. A "living world" is only needed if you travel through the same regions over and over and over. Original Sin is more a classic "linear" experience with hand-crafted encounters for turn-basedc combat. Respawning mobs would just be a huge annoyance. Nothing is worse being forced to fight against a level 10 mob while you yourself are on level 20 for no apparent reason. That would seriously devalue the core idea of this game and on top of that it's absolutely unnecesary.
If you guys want to have more combat replay the game or wait for mods/expansions with new content.