There are dead bodies on Mount Everest because some are too high on the mountain for retrieval to be possible. Other climbers are presumed dead, but their bodies have not been located. Some families of deceased climbers have requested that the bodies of their loved ones stay as they are.
As soon as a body can be reached, it is brought down from the mountain for burial. It is against Nepalese law to leave bodies on the mountain, even if the family requests it. For the bodies that remain on Mount Everest, the cold temperatures are actually perfect for long-term preservation of the corpses.