Why did Egyptians use pyramid shapes for their tombs?

When you observe something, that is science. When you ask why, that is religion. The science is that tribes all over the world began making pyramids within a single century, and they all used the same pyramid shape. (Tribes in northern Europe thought it looked like a tree.) We conclude that they were all looking at the same thing, and it was shaped like a fire, or a tree, or something rather similar.

Egyptians happened to have remarkable skills in mathematics, but the other tribes didn't. So the reason must have been religious inspiration, not mathematics.

If you would like to study the scientific analysis of this, here is an ebook investigating the history. It is very long, and it is still in progress.
www.saturniancosmology.org/