Hmm.. don't people tend to borrow against their houses? If housing prices fall 10-20%, people with poor risk management might get effectively margin called, forcing them to liquidate, which becomes a contagion event.
When people say "you should buy real estate as an investment because you can't live in your brokerage account" and then do crazy stuff with their equity that they wouldn't even dream of when investing (e.g. Boglehead investors who HELOC for some net negative remodel) it really blows my mind...