The only difference is that in the D&D earth Babylon, Rome, Macedonia, tenochtitlan, Paris, London are all sitting on top of cleared out mega dungeons. The impetus for empire is the need to support a powerful upper class when you can’t just send some 20-somethings underground to kill an ogre and bring his gold into the economy. Then, on the periphery of your empire some group of “heros” finds another vast complex and yet another middle of nowhere farming village is on its way.