But that's not _why_ it is vast. The vastness comes from three main things, I think:
1. the number of distinct procgen systems. Creatures have procgen phenotypes and personalities. The landscape, biomes, and simulated geology/hydrology. Biomes. The civilizations and simulated history. The names of things. Religion, Instruments, cultural songs and dances, masterwork items and treasures, legendary forgotten beasts, and a bunch of stuff that doesn't come immediately to mind.
2. the inter-relatedness of the procgen systems. In many procgen games, the different systems do not affect each other. Systems that affect each other multiply the possibilities. And if they all affect each other, it's exponential. They all multiply each other. It's hard to describe how hard this would be to build in a way that wasn't just a mess of nonsense, but they did it by focusing entirely on the underlying systems and the UI was impenetrable ASCII with baffling UX. That has changed with the steam edition.
3. This game was written by two guys over the course of more than a dozen years funded by donation. It is an exceedingly rare and towering artistic achievement. This game should not exist, and will almost certainly never be surpassed (at least not in the same artisanal way).
One of the goals that was mentioned as a north star was that they wanted this game engine to be able to generate any fantasy story that has ever been told. But that none of those stories would be explicitly coded in, the world was just capable of making it happen.
You know, I probably could have just linked to their roadmap. The game is only halfway complete by the creator's opinion: https://www.bay12games.com/dwarves/dev.html
That will give you some idea of the INSANE ambition of this game.