Insuring all those fixed rate mortgages is very expensive. The US has never actually had to do so, the closest being the 2008 Fannie Mae and Freddie Mac recapitalizations which cost $238B in loans that were finally paid off in 2022.

In Europe a lot of the government fiscal crises were a result of small countries having to backstop giant cross border banks that happened to be headquartered there, and there is little appetite for tighter fiscal union.