I have a theory about this, you may find it interesting.
If we excavated a barnyard a thousand years after it was buried in silt and we knew nothing about the planet, say we were archaeologists from the far flung future, we might make quite a few mistakes about the organization of the place.
Our future fossil diggers might conclude that the farm was run by cows, chickens, dogs and pigs. After all, we would find their skeletons there.
We wouldn't find the farmer's skeleton, however. We might not know that is because when the humans who ran the barnyard died, they were conveyed away to be buried in a special place far away from the barnyard. The animals were buried there because they were beasts of burden but the farmer is regarded as special. He gets a special ceremony, a special suit of clothing, he is buried as part of a complex ritual to put him in his final resting place. Nobody buries the farmer on the other side of the fence as the animals are buried. The farmer is special.
I have a theory that when melonheads died throughout the ancient past, they were conveyed away from whatever place they perished in and buried in tombs specially constructed for them. Sometimes we find these places but they are very, very rare to discover compared to the far more common remains of their slaves.
Perhaps there are tombs beneath the ice in Antarctica that stretch for miles underground where the melonheads were buried, when this region was a tropical paradise that housed the melonhead capital. Maybe they are now submerged on the ocean floor miles down.
I think when a melonhead died, it was considered a "human" and due special respect and treatment as well as an elite resting place. Slaves and various worker species were not regarded as deserving of much beyond an expedient burial and therefore are far more common - we find them used in the dedication of tombs and monoliths, often buried alive simply as a token gesture to commemorate completion. I think the melonhead command structure was obsessed with seeing to it that no melonhead was ever buried alongside these primitive creatures and they went to great lengths to always spirit the body away to these special "other" places.