Having a centralised system means that there is one point of failure. If a group of hackers decide to hack that and succeed, they can steal, modify or purge information.

That's why banks, financial institutions and basically any company trying to protect information are building layers after layers of security (firewalls, encryption, access controls, backups etc..). Nothing is unhackable, specially in a centralised environment, and the question in our field is not "will I get hacked?" but "when will I get hacked? And am I prepare when it happens?"

The blockchain, being a decentralised ledger, has multiple point of failure. Meaning that a hacker will have to hack every single ledger at the same time to be able to steal or modify that information. Right now, it is not possible without a massive amount of computer power to break the encryption used on whatever blockchain. It will probably happen when quantum is here but right now blockchain is the most secure technology out there.