There aren't all that many geometric shapes to use when you want to build a large structure without modern construction materials, i.e. steel. Think about it... What were they going to build, a sphere? A square would be too heavy and not stable at all. A pyramid is stable, and the load is distributed evenly while the entire structure gets lighter as you go up. It's a logical thing to do, and prehistoric/copper/early bronze age people were just as smart as engineers today, even if they couldn't perform the geometry and arithmetic on paper.