The 2007-2009 mortgage crises has many reasons for its occurrence, in which, pure greed and intentional deception would have to be amongst the foremost of those reasons. In order for mortgage lenders or for banks to make money, they must either create loans, and/or have investments which will materially improve their bottom loan. When it comes to initiating loans these become upon their inception both an asset as well as a liability for a bank, however, although many banks and mortgage lenders create loans to consumers and businesses, a much smaller subgroup actually keeps those loans on their individual books, with the majority of such, packaging their loans and selling them in securitized packages to investors, here and abroad. Not too surprisingly, mortgage loans that are not sold, are typically looked at more carefully and scrutinized much more since the lender of such, must make a profit on these loans by being in a position in which their cost of money is less than the monies that will be received from the loan, while also budgeting for a certain percentage of bad loans. On the other hand, lenders that make it their policy to sell and package their loans are in almost an entirely different business, which is essentially packaging their loans in such a manner that the selling of them will generate a nice profit.
Investors of all types, whether individuals, businesses, or pension funds, need to or strongly desire to make money on their portfolio of investments each and every year, and in an era of historically low interest rates, this is quite problematic to do so when investing in basically bond-like investments, which mortgages typically represent. Therefore, when a securitized mortgage portfolio is put up for sale, with a return that appears to be rather good, along with the basic understanding or premise that home prices don't ever seem to fall, along with mortgage defaults trending at low rates, the easy conclusion to make is that the investor making this decision to buy these securities is going to make some good money with a minimum of risk. The problem with this theory, goes back to the very beginning, which is, if housing prices do fall because of a recession, or leverage that is imprudently high, or whatever, that presents a significant problem, along with the additional problem that if the people owing money on their mortgages are not credit worthy or are problematic to begin with, that securitized loan is going to be a very poor investment choice with disastrous consequences.
The basic issue that you have with mortgage loans, is if the mortgage broker, or the mortgage lender, or the loan officer in general, is making his real money via commission on the value of the loan and/or fees involved, along with the fact that their management also gets a percentage of the business so generated, it takes no real stretch of imagination to quickly realize that more loans will be issued with lower qualification points being necessary from applicants or even outright fraud, especially if the accountability is obscured. This makes for wonderful bonuses for loan originators, and wonderful profits and bonuses for banks or their equivalencies that are creating the mortgages, but a disaster for those buying such loan packages, and a disaster for whomever is the person or entity holding the bag when it all collapses which it inevitably does.
When money incentives are setup in such a manner that the more "work" that is approved and loaned, without any apparent direct blowback for lies or deceptions or prudency which creates the loans in the first place, the more liar loans and the more liar profits will be generated, ultimately ending rather poorly and dramatically for those not in the know.