OT Postman, Blade......
""""What would be a bank's function in a transaction that was all cash?"""""
Instead of foreclosure many people will simply deed their house back to the bank. If the loan is "upside down", meaning that the original mortgage debt is more than the house value at some point the bank has to realize those loss's. If the the banks capital structure is not strong enough (very common) to take the right down at this point, the bank would just as soon as sit on the vacant property in hopes that the property either appreciates or it's in a healthier position someday.
My best guess is that the banks have learned that the longer they clog up the system the better the chance that the taxpayers will assist in this problem.