Mummies have been found in dozens of pyramids. They are mostly destroyed or partial due to damage during looting. Pyramids are basically giant "Rob me" signs, so it's really not weird that they were almost all looted. There are plenty of Egyptian records noting the pyramids and other tombs had been looted. It was not only pharaohs that built pyramids, other nobles and officials had pyramids and mastabas built as well. It's not like egyptologists just decided that pyramids were tombs one day and that was the end of it. There has been well over a century of research that supports that conclusion.