From what I understand there are two factors here. First, “heroic” societies (societies that put a premium on violence and the ability to wield it), while comparatively less common than non-heroic societies, tended to celebrate leaders and bury them with their weapons or decorative replicas of them, while other societies did not. Second, there seems to be a bias in a lot of historians and archaeologists to focus on heroic societies rather than the more common agrarian and hunter-gatherer societies that existed contemporaneously with them.