This is, IMO something that has infected modern mainstream TTRPG: Monsters hardly run away, even if a fireball just killed half of you. One thing B/X AD&D got right were the heavy use of Moral Checks, e.g. Check their morale on first death in enemy party, when half have died, etc. In fact, fighting is deadly and scary. And these morale checks differentiated undead as that enemy that knows no fear and had no morale checks, unless forced upon them by a Cleric's Turn Undead.