- "<ontoclasm> a meat ration is 5000 nutrition, which is 1666 turns of sitting doing nothing. so if we assume the average guy would eat three of those a day while being semi-active, that puts a day at around 3600 turns... so that means the world ends 55555.5 days after you enter the dungeon <ontoclasm> which is about 152 years"
Officially, it's just a gameplay mechanic, but let's think about this. The gods of the Dungeon Crawl world are very active within the dungeon and we have no reason to believe their influence doesn't spread to the world outside the dungeon too. Additionally, most of the gods are extremely bloodthirsty. Even playing as a follower of Elyvilon, the supposedly pacifist god of healing, your game will play out basically the same way it would under a god like Mahkleb the demon god of destruction, with you slaughtering every living thing and destroying every non-living thing you come across. That sounds like a recipe for a Crapsack World to me!
This is why leaving the dungeon without the Orb is considered a loss even though you survived. The constant wars between the gods are going to rip the entire universe apart in just over a century and a half and the Orb of Zot is the only way to stop it.