i think you got the cause and effect wrong - banks are run securely because it's made to be very difficult to steal money. And stolen money gets tracked (if you did steal a large amount) by anti-money laundering laws, which makes it hard to spend it.
Why is banks' attack surface small? Why is all these other "systems" in place to make stealing money difficult?
Why isn't the same happening with stolen credentials, or data?
https://www.bbc.com/news/business-64240140.amp
> He said Barclays told him it would do an internal fraud investigation which later resulted in Mr de Simone being held liable for all the losses.
> "They could not identify a point of compromise from the back end - to them it looked like the pin had been entered.
> "The only thing they could suggest was that someone knew the code therefore it's gross negligence on my part apparently.
In the olden days banks used to have decent security. Once you gain access to the account, to pay a new payee you'd need your bank card and pin number to do the 2FA. Now it's all the same phone.
> After eight months of evidence gathering and dealing with the police, an investigator at the Financial Ombudsman Service (FOS) upheld Mr de Simone's complaint against Barclays which now, if it disagrees with this, has the opportunity to ask the ombudsman to examine the case.
They are just trying to wear him down.
Bank customer data breach is not that rare.
Money in bank is "secure" because they are cross checked with counterparties. Their data leaks like everybody else.