Banks sell mortgages to people who are statistically unlikely to pay them back, especially in aggregate. Sure, some people who are perfectly capable of paying it back will gain access who didn’t have it prior. But on average, they’re risky loans. And they tend to be predatory. Lots of them involve teaser rates so that payments are low at first and the skyrocket later on or have giant lump sum payments.

A Mortgage Backed Security is an asset made up of thousands of mortgage contracts. So you invest in them and their value goes up as the value of the property goes back and they stay current on payments. Ratings agencies will rate different contracts depending on how risky they are. The 2008 crisis was caused by these shitty mortgages getting packaged alongside better mortgages and sold as a whole new product, which was artificially boosted by ratings agencies. So people were investing in fundamentally unstable assets because they were running out of more reliable ones and capitalists can’t help themselves