Mineshafts, a place that's known to have many recourses, are known to be abandoned, yet no one knows who built them and why they abandoned such precious places...But I wouldn't make a theory if I haven't discovered it, would I?

Instead of giving you clues then the answer, I'm going to do the opposite (Rebel confirmed XD). My answer is: the Villagers. I know that I am addicted to Villagers and maybe that's why all my answers are related to them but just think, you will realize how questionable they are.

First, each mineshafts have a starting room, which is a large room with a flat dirt floor , why is that? Well, Villagers built that room for a farming purpose; also, from that room there are four exits. My thought is, they planted crops there and started mining ores by making exits. However, we don't see any water in that room, but that doesn't mean my opinion is wrong...they carried water with them in case they dig too far away from the room and run out of food. That also explains why you often confront dirt while going through tunnels. "How do you know they planted crops in the dirt room and tunnels?". First, there are beetroot, melon and pumpkin seeds in chests. Second, mineshafts are consisted partly of dirt and they wouldn't make it dirt for fun, that are solid proofs.

A theory can't be that plausible if there's only one evidence... So I'll give you another one ;). It's simple, the loot in chests. If you notice, you'll get confused: why does anyone leave ores in the chests when that's what they're aiming for...unless, they have very little inventory slots and aren't interested in them! Analysis time again.

• Players? Likely no, they have 36 slots and loot chest only have 20 items in total . Yes, they may have other items that are already there in their slots but WHY did they decide to leave precious things in the chests? They came down here for those stuff, too. Therefore, players didn't built mineshafts.
  
• Villagers? Likely so, they only have 8 inventory slots and pickaxes, hoes, foods, swords, their own stuffs already took 5-7 of their slots. What is their own stuff?, that's the stuff based on Villagers' profession.
  

Some professions' items took up their whole inventory, so they have to sacrifice some of the stuffs and hope that some Villagers will collect them. Thus, only Villagers prefer Emeralds, rather then Diamonds, which is why you never see Emeralds in loot chests.

There you are, Villagers are full of surprises :D I feel like this article is too long so I'm gonna talk about why they abandoned mineshafts in a specific theory. Hope you enjoy, feel free to argue!