There's a world of difference between "We misjudged the long-tail risk of the instrument we rated" and "We straight up lied so that we could keep getting business from the banks."
Half the mortgages in some of those AAA instruments were issued to people with income at minimum wage, or with credit rating in the 500s, or to people buying homes that were never even looked at by underwriters. This isn't a "Whoops, our math was a bit off, we didn't expect the collateral to drop in value for a bit."
This is straight-up fraud, and the people carrying it out should have been taken out back, and had their shirts taken away from them. We need more, not less personal responsibility from our professional sectors.
Fraud is the only thing that caused the financial crisis. Period.
Fraud from the top on through the bottom. The guy lying about his income to the mortgage broker who knew the dude was lying (and likely told him how to do it effectively), to the ratings agencies lying, to the wall street banks lying, to robosigning, etc. etc. etc. It was outright fraud every step of the way and almost no one who participated in that market has clean hands.
I am sure there was fraud as well. But fraud doesn't explain the financial crisis. Investor complacency, untested new product, excess leverage and reliance on short term funding, too much liquidity in the system, interest rates too low for too long, these were the fundamental reasons.