Several reasons:
1. the bank does not want to reward the borrower by allowing him to get the property at a discount. ie the the bank bids 50k and the borrower bids 55k, then the borrower gets the property for less than the original balance owed.
2. most loans have been sold so that they (the lender) have a contractual servicing agreement to bid the balance.
3. many loans have pmi so that the lender gets paid in full. if they bid less, then pmi pays less.
4. the many states have laws that require the lender to bid market, since market is hard to define in a foreclosure, they bid the balance to stay safe.
5. left hand right hand. the legal team does not always get the asset management team on board until after the sale.
6. many states have right to redeem the property, a lower bid means that they can redeem for less.
7.you never know if someone will step up and pay.
8. accounting rules for banks and potential losses are complicated but in general, a lower bid is usually not good for the bank where as a REO can be "fudged".
9. accountability, banks are owned by shareholders, selling below market would be a breach of fiduciary duty to the shareholders. I know, you think that they lose more money but the issue is sticking to the process to avoid the appearance
10. as a former asset manager, we actually averaged 103% above the balanced owed on our REOs so do not assume that the value is going to be less than the loan, marketing time is critical.
the list goes on.