For the ancient Egyptians the body of the deceased was really important, as they believe they would keep it in the afterlife.
At the same time, the Pharaohs were buried with a lot of treasures, so their graves were at the risk of being plundered, or robbed and destroyed, and perhaps even the body would be damaged then.
So to protect to body and ensure the treasures would still belong to it, they decided to hide their tombs.
