When we can't understand the science behind something in this world, we make up mythological entities that we can relate to. We personify the forces of nature that mystify us, using our boundless imaginations to comfort us and make us feel like we have some control over these things that are much bigger than we are.
People disappear when they die. Their voice, their laughter, the warmth of their breath. Their flesh. Eventually their bones. All living memory of them ceases. This is both dreadful and natural. Yet for some there is an exception to this annihilation. For in the books they write they continue to exist.