Well, the first thing that comes to mind is that that's part of saving resources used by the game. Even though it's a corpse, it still has to be rendered, thus consuming system resources.
Yeah but that's what LOD's are for, - In games where you can .ini your way into permanent corpses I've never seen performance degrade from keeping them around.
If the game can draw a crowd of them with AI and collision enabled while they're alive, it can certainly draw them as non-colliding null objects.
