I make this point because these people aren't super rich wall street people above the law. And plenty more than one were caught and jailed for it, I've found 4 examples on the first page of my bing search. Stewart is saying only one bank exec went to jail, but bank execs weren't the ones doing this mortgage fraud.
Some "Wall Street" banks do sell mortgages directly and would employ these mortgage offices, but a lot banks don't like Goldman Sachs, the former Lehman Bros.
The question is whether big commercial banks were complicit in the fraud of their lower level employees. You'd need to find an email or someone willing to testify that the banks knew that the loan offices in podunk were encouraging fraud. That is an extremely tough case to prove without solid evidence.
Also, this kind of document fraud was only a minor part of the financial collapse. The big mistake, which Stewart briefly touched on, was that the rating agencies massive fucked up. The financial risk model they used didn't work.