Jack McGillivray, Intro, Question #5

I have always heard about the housing bubble that caused the recession, but I never knew what it was or how it could cause such a severe economic downturn. The way it was explained in the chapter made it very easy to understand. I never knew that it could be beneficial for bankers to give loans they knew were risky or even sure to default.  I was also unaware that a debt or loan could be sold off. To me it seems strange that a debt can be transferred with no input for the debtor.  I am curious as to why economists did not anticipate this crass, when in the book it seems very clear that this was inevitable due to the loaning/borrowing practices.