Answer:
Because they believed that the structure should be the house of their deities, and where their rulers should rest, among the gods.
Step-by-step explanation:
Like several ancient societies, the Aztecs and Egyptians built pyramids for religious purposes. Even if these civilizations are separated by the distance and the time, they share similarities. The idea to connect the terrain world with the gods was something shared by both societies, and the pyramid was the structure responsible to do so. An interesting point about the pyramids of both civilizations is that they were built following severe astronomical rules. If it's the way to talk with gods, and the connection of both worlds, so the structure should be correctly arranged with the stars and the sky. No matter if it was bald like the Aztecs pyramids or pointed like the Egyptians ones, there should be the place where the gods would live, where they must pray, and most importantly, the resting place of the king.