Risky investments aren't necessarily bad investments. Low interest rates give businesses more runway to operate investments that might take a while to show returns. A million dollar loan at 10% for an investment means that it needs to return into ~$80k a month to break even. At 2%, that same investment only needs a ~$16k monthly return. That's a huge difference in runway needed to start generating cash flow.
See Antoinette Schoar's (MIT) work: https://www.nber.org/papers/w20848
It makes for a good news story that regular Americans understand and frankly want to hear (the banks were bad). But if you really dig into the reports, e.g. Financial Crisis Inquiry Report, almost the only fraud comes from misreporting of income which is essentially consumers lying. There could be other documentation issues, but underwriting almost entirely depends on credit score + income, so they're fairly trivial.
There are diminishing marginal returns to this, and when rates were already close to zero it is hard to imagine going to zero spurred much more than meme stocks and YOLO gambles.
the leverage for nonexistent (or overclaimed) mortgages were
leading to a crisis of confidence where credit dried up because banks didnt trust each other
got to watch for that. know the contents of your securities