The Giza pyramids are not the beginning of pyramid history. The evolution of pyramid building is directly traceable from mastabas, to stacked mastabas, to the first attempt at pyramids, to the great pyramids, to the later pyramids. All of which were used as tombs. It doesn't make any sense that the Giza pyramids, which are smack dab in the middle of that history, would have a different purpose. The fact that mummies haven't been found in the Giza pyramids is just something conspiracy theorists use a jumping off point for wild theories that they had some other mysterious use. All archaeological evidence points to them being tombs.