The pyramids are very explainable. Read Revelations 21, then read it again and again until you figure out what John is describing in his vision of the new heaven coming down to Earth. Hint: key is in 21:17 where he describes the size of the ‘city’ being 12,000 stadia on each side, and as wide and HIGH as it was long. He is describing a 3 dimensional city that is 1,308 miles wide, long, and high. Our limited experience and precepts tell us that a ‘city’ is generally described in 2D format, or square area. What geometric shape is as long and wide as it is HIGH.? Yes, it could be a cube - but given the ancients propensity and disposition towards building similar pyramids all over this planet I’m gonna bet that’s what John is describing.

Who and why were they built? This is alluded to in Genesis 6 when the sons of God decided that the daughters of humans were beautiful - and they took of them that they wanted and had children with them. The children were the ‘heroes of old; men of renown.’ Kinda sounds like the offspring had supernatural powers and abilities akin to Greek mythos perhaps? Why the pyramids? Perhaps they were homesick and trying to recreate that which they had chosen to ‘fall’ from.?