system
Given the following post, respond as if you're commenting on Reddit:
human
[General Fantasy Dungeon Crawl Video Games] Why are there so many freaking dungeons? Why are they so sprawling and labyrinthine? Why are they all seemingly abandoned?

*Watsonian aside for context: I play a lot of idle dungeon crawl RPG games. Examples include Clickpocalypse 2, Idle Sword 2, Dunidle, Soda Dungeon, and others. They're games where you select your players, buy gear, upgrade gear, and sometimes deploy things like spell scrolls. What separates them from other dungeon crawl games is that combat, healing, etc. occur automatically. You upgrade stuff, go do some work, come back an hour later, and buy upgrades. Your guys basically progress through an endless series of dungeons.*

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Okay, so it is my understanding that traditionally, a dungeon is a semi-finished area underneath a castle, a monastery, a cathedral, or another large structure in a world inspired primarily by medieval European folklore and history.

Traditionally, these areas are generally used for holding prisoners. These could be outlaws who hunted illegally on the king's land or something, they could be prisoners of war, whatever. Sometimes there are torture chambers and such, sometimes not.

**Why are there so many unexpectedly, impractically large and sprawling dungeons, which are by all accounts completely and utterly abandoned, and consequently infested with various monsters?**

Much of the time, they don't even seem affiliated with any castle or monastery. When they are, the building itself also appears abandoned. There are no prisoners or guards or anything. In fact, there aren't even any rooms that look like they were ever designed to be holding cells at all.

Who built these and why? Why did they abandon the dungeons, and/or the structures above said dungeons? Why are they incredibly massive and sprawling, often dwarfing the castle they're attached to? 

Why did whoever abandoned them neglect to collect their gold and treasure before doing so?

Why are there so many abandoned? I mean, abandoned structures, sure. But why so many of them? Was there some kind of apocalypse or something?
gpt
The dungeons were created by many different cultures and species throughout the millennia.  

In a few cases they were literal dungeons - underground areas designed to house criminals and political prisoners.  They were created to be labyrinthine and extensive to foster a sense of hopelessness in its occupants and to prevent escapes.  

Sometimes they were created to house dangerous monsters - creatures sacred to a specific religion, unholy abominations made by magical experimentation, exotic beasts obtained in faraway lands.  They may have been kept as sacrifices or to sacrifice to, maybe they couldn't be killed for political/social reasons, maybe they were literally unkillable.

Once there were structures above these on the surface.  They have since rotted, crumbled, and/or burned to the ground.

Other "dungeons" were built by races who live under the surface, like dwarves, goblins, dark elves, etc.  These were their cities, homes, outposts, and trade enclaves.  Powerful wizards and other eccentrics likewise might make strongholds for themselves underground.

They are all called dungeons because that's a generic term for a constructed underground structure, distinct from natural tunnels like caves and volcanic pipes.