" No machines have been found in the archaeological record to support these assertions, but there is an abundance of circumstantial evidence that leads to such conclusions. Are the machines still intact and lying under the desert sand? Or were they removed completely from the areas? Or could it be that all this evidence points to an earlier civilization that suffered a cataclysm of such magnitude that much of what existed was destroyed, and what remained was susceptible to erosion, decay, and corrosion, and slowly disappeared over a long period of time? This brings us back to Robert Schoch's evaluation of the erosion pattern on the Sphinx and the Sphinx enclosure. He claimed that the period of time when sufficient rain fell in Egypt to cause this erosion was seven to nine thousand years ago. Is this sufficient time for ancient machines to turn to dust and blow away? It seems incredible to imagine, but there is reason to suspect that this could have happened.
[...]
If we follow the idea of an older civilization, therefore, the pyramids would have already been there before the first dynasty of the ancient Egyptians. The Great Pyramid was, most likely, the zenith of construction on the plateau, and the other pyramids were likely built before it was. Yet something happened to the culture that built the pyramids, and when Khufu came on the scene, he naturally chose the most impressive structure--the Great Pyramid--as his own, and his heirs took turns in claiming the rest. What event could have brought death and destruction to this ancient civilization that is referred to in Egyptian lore as being inhabited by gods of the First Time? "

― Christopher Dunn , Lost Technologies of Ancient Egypt: Advanced Engineering in the Temples of the Pharaohs