OK, you have a $750,000 mortgage on a $1,000,000 house, plus $250,000 in equity.
Then the price declines and you have a $750,000 mortgage on a $700,000 house, plus $175,000 in meaningless equity.
You can sell the house and pay the bank $50,000, screwing the bank, or you can deliver the house to the bank and pay them $0, screwing the bank much harder.
Meanwhile, you've lost... the opportunity to take out a second mortgage against the equity you had in your house? What was the equity doing for you?
That equity was only useful to you if you had the option to sell your house. If it was your only house, the equity concept wasn't all that meaningful, as applied to you, to begin with.
Try changing the numbers. You have 100% equity in your house and the price falls by 120%. (What happened?) Are you getting screwed? How?
You have 70% equity in your house and the price falls by 100%. Are you getting screwed?