> Why would they sell
In many cases, they don't, they just walk away.
Would you stay in a 500k 2000sqft house that you have no equity in, when a 300k 2000sqft house was for sale next door? In a non-recourse state?
And even for principled people, when you lose your job or fall behind on something you overpaid for, it's really easy to just walk away.