That is, it seems like nowhere else has the idea of purification through suffering really taken hold at such a widespread level for so long. I'd argue its because its just the flipside of the Prosperity Gospel. If you assume that one's physical conditions are a reflection of one's spiritual choices, then your first move when someone says "I'm in pain, please help me with my pain" is to ask "What have you done to deserve this pain? I won't remove God-given pain; if you deserve it, it would be a sin to do so", instead of "Here's something to make the pain manageable while we figure out how to remove the source of the pain".