Why were pyramids built in the desert and never on the ferile land beside the nile river?

It was because it was a dry environment. If they were built on fertile land than the moisture would get in to every crevace and decompose the stone, the pharaoh and make all their valuable metal/gold belongings rusty. Before the Pharaohs were put into their tombs, they were left under salt piles up to 42 days to draw all of the moisture of of their bodies so that their flesh wouldnt rot when they were mummified. Hope that helps you!