This is not a entirely valid assumption. It has not been proven that these civilizations only had stone tools. Attempting to portray them as primitive people with only stone tools is highly reductive given the level of sophistication of the structures they managed to create. They may have had sophisticated chemical processes to shape and form rock structures. A simple proof is just by looking at these structures and seeing that even today in 2021, we are unable to form equivalent structures ourselves using modern cranes and rock shaping tools. It is terrible that the knowledge was destroyed by burning their books-khippus and extinguishing their stone masons, technicians and scientists.