All banks are effectively running a huge database at the core, yet that's not a common problem. They have procedures, they have backups and they have human oversight. The last element is key, if somebody manages to hack into the system and send all the money into their own account the bank's administrator can detect the problem and say "well, that's clearly not supposed to happen, let's roll that back".

On a decentralized, trustless blockchain you have no such thing. So you either end up hard-forking like Ethereum did after their DAO debacle (but then are you a decentralized trustless system anymore?) or you continue normally and basically the people who lost their money are screwed.