https://en.wikipedia.org/wiki/Sedlec_Ossuary
This article almost gives the impression that monks did it because they were bored. Or wanted to create a tourist attraction. Seems kinda weird for devout 16th century Christian monks to desecrate 70,000 human corpses "just because".
Well it does mention the black death providing a steady supply of corpses, but that still doesn't explain the decision to exhume graves for an art project.
Maybe there's more to the story? My thought is perhaps the darker parts were left out of history, and most of it was extremely dark so we are left with no explanation
by MolecCodicies