Well. Except they are. In the US not only at the federal level but also at the state level:
- https://en.wikipedia.org/wiki/Deposit_insurance
- https://www.fdic.gov/deposit/
- https://en.wikipedia.org/wiki/Federal_Deposit_Insurance_Corp...
- https://www.cnn.com/2020/04/17/business/bank-earnings-defaul...
Here's the list of banks that have failed in the US and for which their deposits entered receivership managed by the FDIC; looks like a handful every year: https://www.fdic.gov/Bank/individual/failed/banklist.html
Yes banks can fail and they can go bankrupt, and if that happens to Nubank then this acquisition goes to hell which makes the Clojure ecosystem weaker. I give that to OP.
But saying that banks are "very very" fragile institutions I think is fundamentally wrong.
And the function of the regulator is to stop and DISSOLVE the bank BEFORE it goes potentially too deep into the pocket of deposit insurance fund in case of running astray. So this is a social instrument but again does not solve the institutional risk problem. Maybe, just maybe makes some banks more paranoid, but they still blow up at very high pace.