Hi, about the reason of why ancient Egyptians build pyramids the answer can be religion believes, such as they build pyramids as tombs. and about how they build pyramids there are some theories. As the pyramids are made of huge stones it looks hard for human to build it, and there might be a mystery how they built the pyramids. Some theories say that some aliens helped human for building pyramids but there is no evidence for this theory. Anyway every mystery will be solved a day and I hope this one solve very soon. And here is a video about pyramid construction.