Personally, I wondered why there were any mortgages with "stated income" and "stated assets", in which the mortgage company didn't verify income and assets - liar's loans, as they were called.
For example, Columbus, OH went on a spree where the city worked (at behest of I think the mayor IIRC) with banks to qualify financially poor people in the Columbus itself for mortgages in outlying towns, such as Reynoldsburg, OH where my parents are. It didn't matter what kind of income they had. The goal of the politicians was to get the poor out of Columbus and make them an SEP - "someone else's problem". And it worked, but it also saddled those outlying towns with (a) a higher housing demand and (b) people that were destined to be foreclosed on.
Case in point: One young lady in my parent's neighborhood graduated high school, didn't have a job, but got a mortgage for a $150k house. There was no way she was going to be able to make the payments, and was eventually foreclosed on, devaluing other houses in the neighborhood as a result.
As I said...there were many different factors. The above was one of them, and sadly those politicians will never be held accountable for that kind of fraud - fraud that really did make contributions to the over 2008 financial crisis.