I have heard the line several times that the housing crisis was mainly caused by the government forcing banks to give loans to poor people who defaulted then.
However, with their typical junk rating they're a small portion of the market, interesting only to investors with an appetite for risk.
The systemic trouble started when instead of junk those loans were rated AAA, which then opened the door to pension funds and institutional bond funds.
I have money invested there because I feel it's a better social use than gambling on the stock market. I see good solid 5% returns. Most of the loans are repaid quickly.