If you're referring to the coinbase wallet, that's on the blockchain.

you're sending it to a custodial account owned by coinbase, not your own personal Bitcoin account.

It's the same thing as having a bank account and giving all of your money to the bank.

You're giving up absolute control of your funds for convenience, absolution of responsibility and imagined security.

That's how broad societal financial operations still work for the moment. centralization isn't always nefarious, but it is largely archaic at this point.