The mortgage side of the banks had a fell funded government monolith that set up a willing market for its junk. Why would they worry about quality? The amazing thing was that the investment side of the same banks were buying equivalent junk from other banks, knowing perfectly well what the game was. Is it even possible that they thought that they were the only ones playing the game?
Yes, most local banks could see the fire at the base of that smoke and chose not to play with it. They never stopped selling mortgages because they knew what the quality of their portfolio was and they weren't under water.