Heaven. Then vs. now?

Let's say the idea of God and afterlife and heaven and all of that is made up, k? For the sake of this question, let's say the idea of heaven is for a security blanket. That said, what did the ancient Greeks, Babylonians, Sumerians... All those guys, why did they paint the picture of such a grin after life? In the Epic of Gilgamesh, a Sumerian story, the after life is described by saying all its inhabitants eat meat of clay and dust; men who were once kings are now slaves. If the idea of heaven is supposed to be comforting... Why was in so terrifying in ancient culture?