You can't run a truth layer on a database. A database has an owner, and an owner can be asked — or paid — to change a number. That's not a bug in someone's implementation. It's the category.

A chain changes the question. Signatures, slashing, and a public ledger mean the number either came from the median or it didn't, and anyone can check which. When a contract is about to move real money, that's the only kind of number worth trusting.

database:  "trust us, this price is right"
chain:     "verify the signature yourself"

the signature is the point

Every value carries the proof of where it came from. Your contract checks the signature, not the vendor. Data integrity stops being a promise and becomes a cryptographic fact.

Data doesn't need a chain because databases are slow. It needs a chain because the truth, once money depends on it, is an adversarial problem.