It's the bank's job to care, if it's in their best interest.
Theoretically, in cases where folks owe more than a house is worth, banks would love for that homeowner to stay in that house, and keep paying the higher-than-market value mortgage. And that's a conceivable option for many people.
But what about those cases where the person is going to move? Whether the reason be job, family, they can't afford the mortgage, whatever. Then there are 2 general possibilities:
1) Bank approves short sale
2) Bank does not approve short sale. Owner walks away from home. Bank spends $$$ foreclosing. Bank sells house for price they would have gotten if they had accepted the short sale.
In case #2, the banks loses money. Yet, they are allowing cases like this to happen.