What’s the lore reason for only dungeon skeletons dropping bones?

The skeleton is so old that it just dissolves when the magic holding it together (its HP bar) is gone. Angry Bones are held together by the magic of the dungeon, but their hp bar is their rage. When you deplete their rage they just become normal bones instead of angry ones, hence dropping bones.

Disclaimer: This is pure and utter BS but it kinda sorta makes sense in my head

Their anger keeps the stress out of the bones and in their skull, that’s why the angry bones don’t drop skulls

the guide realized that if he let you make necro armor too early then his truman show style simulation he’s using to torture you for eternity won’t work as well. the guide is your prison guard trapping you in his terrarium making you do tasks so that he can eventually become the next lord of the moon. the skeletons in the caves underground do drop bones but the guide just never told you how to pick them up

I always justified it as the caverns’ damp environment resulting in the skeletons having very weak and poor-quality bones.

I just realized, can’t you find pianos underground and then shimmer them?

the non dungeon ones are actually just made of milk