They quit and logged off for a long long time

According to the minecraft historian Snarkius Maximus:

In ancient times. the original populators of the world did explore until all was unalterable darkness (33,554,432 from World Home)
but girding themselfs with diverse and eldritch magic pushed farther until the world itself ended 9,223,372,036,854,775,807 blocks out

Undaunted even by this limit, they sought further knowledge and open gates into Hell itself, and went forth conquering and to conquer until – here too – World's End was reached.
And in their explorations found and came to know much arcania, portions of which would fuel their still burgeoning civiliztion.

Some items – rare and wonderous, the Ancients learned to combine and (in combination with further workings, the secrets of which are now lost) they opened yet more gateways, for the Ancients were not those to accept limits upon themselves…
Through these new gateways they proceeded into a land of perpetual night, conquering great wingedy beasties and traversing vast bottomless chasms in their continuing search for more glorious shiny bits.

Yet even here (370,727 from the correspondance point of World Home) they found themselves constrained by hard limits .

A dark time followed as decadence and despair stalked the lands.
But hope was reborn that a petition to the all powerfull Program Code for new toys to amuse the populace might offer solace.
And so it came to pass that the Ancients concieved and implented the first Mob Vote…

What manner of creatures were proposed is lost even to the greatest of sages, but the results are plain…
The Ancients (unable to agree or compromise amongst themselves) divided themselves into factions which warred and spat upon each other damaging or destroying much that was… or could have been.

Of the many Ancients who began the fight, only a scant fraction survived as degenerate shadows of their once great selves.
Now unable to create settlements beyond a paultry 1,073,741,824 blocks from World Home, these Testificants now spend their lives in simple labors, mumbling of past greatness and waiting for the external motivation of the new apex inhabitants… "

Depends on who you ask.

Some say they were wiped out by a Zombie Plague (hence Zombies and Skeletons), others say it was war (either with another civilization of Ancient builders or some other civilization that existed at the same time), a few others state natural disaster (e.g. famine, volcanic eruption, or flood), or the Wither (the mobestiary stated the first wither was made out of curiosity after someone made a painting). Could also be all the above or perhaps the Ancient Builders left to other worlds after a time of great progress and nothing hugely bad happened (End Cities and what not).

Either way, in my personal theory, the remaining Ancient Builders decided to band together after one of the speculated disasters (I think it was a combination of War, Natural Disaster, Zombie Plague, and the Wither), and built the Ancient Cities underground to work together to ensure their survival and perhaps find a new dimension to flee to. You can infer what happened next as we speak.

Then again Minecraft's Lore is up to interpretation due to its Sandbox nature and there may not even be Ancient Builders in your Headlore. There also might be something Minecraft Legends and Dungeons said that I may have missed, since Legends is a prequel to Dungeons and that is a prequel to Minecraft, but I have only played the original Minecraft.



(Although I had always thought Minecraft could had been post-apocalyptic by the naturally generated remains of what were likely structures that had a use a long time ago, the villagers were more recent and decided to form a civilization, and that the Player is a descendant of whomever used to live there)

can i shamelessly self promote a blog i made about it?