the thing with 2008 isn't that 20% defaulted. it's that the banks built an insurance pyramid scheme to borrow more money and one bank was left holding the bad debt.
Bank A , B , C and D each have 100 loans of which 95% are good and 5% are bad. Each bank is maxed out on loans they can legally, and fiscally move. to make more money they need to insure the loans.
So Bank A and Bank B take 50 loans each 45 good one and the 5 bad ones and get insurance from bank C. This gives Bank A and B the ability to take on 20 more loans each with at least 2 of them being bad.
Bank C goes to Bank D for insurance and gives Bank D their 5 bad loans as well as the 10 bad loans from Banks A and B. this gives Bank C ability to also take on 20 more loans.
Bank D is now holding 190 loans including 20 bad ones and their risk went up from 5% to 10%.
Wash rinse repeat. they did this until one bank had 20% plus bad loans in their portfolio. when it collapsed all the cross insurance collapsed as well. That is why 75% of the government bailout loans were repayable in 4 months. The banks just needed to cover short term costs with the pyramid to each other.