I know it's not popular a design (sleepers respawns when leaving POI), but it is an effective counter measure to prevent people from cheesing dungeons within other games. In other games, after you drag the mobs out far enough, they reset and run back to their original spots lol.
Running into a POI and dragging out all the zombies in order to loot everything shouldn't be a strategic option imo (regardless how immersive/realistic it may be). Maybe there is a compromise out there somewhere...for example, if you run out trying to wake everything up. A horde should spawn outside making it more difficult to run back in....
That way, if you do run back in after dragging everything out...it becomes more risky (risk vs reward!!!)