In order for you to gain money that means someone else somewhere had to lose money. If it didn't, that's a violation of accounting principles that would immediately flag your account.

So if your well-orchestrated hack also drained someone else's account in order to increase your account, that other person would notice, and it's pretty easy to undo bank account transfers.

It's a pretty silly approach anyway since if you want to steal money from someone's account you don't even need to hack the bank, all you have to do is know the other person's bank account or debit card number, which is plainly printed on the front of any check or credit card number they give you. But again, if you tried such a thing, it's very easy for the bank to undo.

As for ransomware, banks are easy targets for ransomware, but banks also typically have frequent backups that they could just revert to if their database of transactions was ransomwared.