You seem to think you’ve found some sort of gotcha. There were plenty of crimes committed in the MBS world. See GS, Credit Suisse, and others. However very few were prosecuted at the individual level.
It's a fair question that I used to ask, but it's a very answerable one. To pick one example (https://www.sec.gov/news/press/2010/2010-59.htm), the SEC believes that one Fabrice Tourre was primarily responsible for the mortgage fraud committed at Goldman Sachs. But either they never referred him to the DOJ for prosecution or the DOJ declined to prosecute him.