Why are there no mobs unique to caves?

I've been playing since 2011 and I've always felt that the underground in Minecraft lacks unique mobs. I propose adding the "Scourge," short, hard-shelled creatures that bore through the deep crust of the overworld, resembling the head of a mining drill.

The worms would move through blocks without breaking them and only certain blocks would be impassable to them. They would expose themselves briefly to land an attack on a player. They wouldn't spawn everywhere and would only spawn from "hives" that the player can take and even use to farm.

Possible Additions

Though their tough shells would act as armor it could also be what allows them to pass through blocks and so without it, they could squirm helplessly. This shell could only be broken using a pickaxe rather than a traditional weapon. Upon the shell breaking it could have a rare "Scourge Shell" drop that could be the ingredient for a resistance potion. Upon one dying they could also have a guaranteed drop of "Scourge Meat".