The banks were at fault because they believed the upswing in housing prices would never end. What were they thinking when they were loaning money to these idiots? That the price of an average suburban home would be $4 million in 10-15 years? If the banks had kept the 20% down rule that was in place for years, this would have never happened.
Instead they were more than happy to give interest only loans. Figuring in 5 years when their mortgages ballooned, they could sell and cash in their profits. It didn't work out that way. You had too many people in the housing market that had absolutely zero business there.
The banks were essentially the bigger idiots, in the bigger idiot theory. People were borrowing money like there was no tomorrow... And the banks were more than happy to give it to them. Because houses never go down in value.
Finally, just like the stock market in 1929 it collapsed, because there was no actual wealth in any of these transactions... Only paper and debt. The result was the same. The entire market collapsed.
Follow me here:
1. In the olden days, banks lent their own cash reserves to customers with the expectation that they would get paid back. During this time banks vetted their customers and their ability to repay their loans.
2. In modern times, banks who originate loans sell those loans to other financial companies.
3. Poor oversight allowed these firms to bundle these mortgages into mortgage-backed securities which they sold to investors.
4. Investors now received the proceeds from mortgage payments and the banks who originated the loans were made whole and no longer had anything to do with the mortgages they originated.
5. The more mortgages the banks originated the more demand there was to bundle them into MBS to sell to investors. As you can see, at this point, the banks originating the loans are decoupled from the ability of the mortgagees to pay their mortgages.
6. When originating banks ran out of qualified applicants, they sold mortgages to unqualified applicants and to their surprise there was still demand for these subprime mortgages in MBS.
7. The ratings firms were also complicit. They took MBS comprised of sub-prime mortgages and gave them AAA credit ratings (for a fee). Rating agencies who didn't go along were afraid they'd be "black balled". How can a bundle of mortgages with mixed ratings somehow be deemed AAA?
Were the banks stupid? No, they made A LOT of money selling these crappy mortgages to idiot investors, they were made whole so no risk to them, and investors like me and you who had no idea what was going on were left holding the bag. This doesn't even get into the credit default swaps which were also a terrible idea.