This lacks an explanation as to why someone would by them.
Because the crappy loans were mixed in with good ones and the rating agencies said those loan packages were all great.
^This
The single most important element to the entire debacle was the fact that the ratings agencies (Moody’s and S&P in particular), did not assess the risk properly (independently of each other) and gave junk such high ratings.
Had they actually done their jobs and dug deeper into the underlying assets like they are supposed to do, they
should have discovered the problem and given the assets the low ratings they deserved and none of this mess would have happened.
There has been a LOT of debate (and accusations of bribery/willing complicity) as to why that didn’t happen, but the fact of the matter is they simply fucked up (and, again, they did so independently of each other). They simply did not understand the complexity of the assets and did not investigate the mortgage
holders, evidently assuming (incorrectly) that the banks/underwriters of those loans had done their own due diligence.
There is a secondary element that helped cause this central problem, which was the fact that banks were no longer doing their own due diligence on these loans. A new industry had arisen; mortgage brokers that were not directly affiliated with any bank. They were supposed to do the background investigation into an applicant’s finances/abilities to pay their mortgages on a regular basis, but because the banks paid them a commission for every mortgage they secured (and because the government insured banks through FDIC), these third party brokers started not to care about the due diligence and gave out loans to people the banks would not have otherwise loaned to had they done their own due diligence.
Iow, it was a tragedy of errors that hinged on incremental cracks in a foundation that no one knew were cracks. Well, almost no one. There were a few (among them Hillary Clinton), but the biggest problem was confirmation bias. No one thought that a few bad apples (once bundled together with a bunch of good apples) could take down the whole structure. That was, after all, the purpose of bundling the bad with the good to begin with.
They just had no centralized idea of how many bad apples there actually were.