Why is a ziggurat in the shape of a pyramid?

The builders did not record their reasons so we can only guess. In all the ziggurats I have seen, the resemblance was quite vague, so it may only be an artifact of modern interpretation. One thing that is not widely known is that the sky looked very different in those days. A lot of things done by the ancients were made to resemble things that modern people have never seen. Here is a long analysis of those records: saturniancosmology.org/