In 2008, the idea was that if you bundle up a large bunch of mortgages, then the bundle will have low risk because the chances of everything failing at the same time is low. The cloud is designed so that resource usage spikes of individual customers can always be served because one customer is very small compared to the whole infrastructure.
However, in some cases, these mortgages/resource spikes become highly correlated.
If every gym member visited the gym at the same time, they wouldn't all fit. Only a small fraction of the members use the gym at any one time, so it works.
Banks would crash if everyone tried to withdraw their money at the same time, but they don't, so the bank can loan the money out.
Only a fraction of the members use the gym at all. If every member of the gym wanted to use it there would be no reasonable schedule to make that possible. ~50% of gym members use it less than 100 times per year, and only ~25% use it consistently.
For banks depending on legislation they have to keep 0/3/10% in reserves, depending on the size of the bank. Which is far worse than most clouds or gyms would ever offer.