I thought about getting into this, but I was trying to keep the metaphor simple. But when you really think about it, the intent of the two systems are pretty similar: a mortgage is backed by a physical asset (a home) that is usually worth about as much as the debt that finances it (if not more), and can be liquidated to guarantee the debt if the borrower defaults. Stock investors have no equivalent physical assets to back their trades, and banks have established the margin-call system to compensate.