A lot things that seem like terrible ideas are not necessarily fraud, at least, not in the legal, criminal sense of the word, because it's not always illegal to make bad decisions with money.
It's not illegal to loan money to someone who can't pay it back--which is the bulk of the documentary evidence that I've seen. If a bank knows I faked my income statement but lends me money anyway, that is a bad decision, but it's not criminal fraud (except maybe by me!).
It's not illegal to sell securities that might lose money. It's not illegal to buy securities that might lose money.
It would be criminally fraudulent to materially misrepresent the facts and risks--for example for a bank to say "we verified the income on all these mortgages", or "I guarantee this mortgage backed security will not fail."
But except for a few rare cases, that's not what happened. It was ratings agencies--3rd party companies--who decided to rate some mortgage-backed securities AAA without knowing what was in them. But again, that's not against the law because the ratings agencies are just private companies expressing an opinion.
NOTE - I'm not defending what happened because it was obviously stupid. And I'm not saying that no criminal fraud occurred, because some did.
I'm just an advocate for people understanding what is the actual state of the law, vs. what seems like it "should" be fraud.