The housing bubble is an "It takes two to tango" type of situation. The banks didn't want to finance people to buy houses that they couldn't afford, but the US. Government persuaded them. On the other hand, no one forced people to buy houses that they couldn't afford. Where exactly where you planning on pointing your finger?

Or, are you referring to banks investing in things that they shouldn't have and betting on people going bankrupt? If so, I'd point you back to the gov persuading banks to take huge risks by financing anyone for a house, regardless of if they could afford it or not.