Eh banks run securely because it’s very difficult to steal money.
Hard currency theft requires a physical attack and “digital currency” is just essentially a spreadsheet that requires a settlement mechanism such as correspondent banking to work.
Banks transfers are nothing more than messages going between different branches and banks there is nothing being transferred other than orders.
The attack surface on modern banks especially large ones is actually ridiculously small since you don’t only need to defraud or compromise a single bank but also the entire system and all other banks which are using it since once the offended bank notices some inconsistency it can issue a notice to reverse any offending transactions.
Also since bank transfers are often liabilities for most banks e.g. when an account in First Capital transfers $1M to someone in First Direct it means that First Capital now owes First Direct $1M which makes First Direct a creditor which is why it will likely quarantine the funds until the transaction is fully verified and settled and even then there still likely going to be a cooldown period to reduce the risk even further.
Most of the security within banks is designed to deal with internal threats since the entire banking system is essentially based on mutual trust which gives individuals even fairly low ranking branch employees the ability to authorize fairly substantial transactions.