Online wallets can be hacked just like self-hosted wallets. There's malware that hijacks sessions, logs your passwords, there's sim swapping, email hacks, etc. Since you access Coinbase on an online machine, the risk of such hack is much higher than with cold storage, which is designed to minimize the chance of hacks.
Then there are counterparty risks. Any centralized exchange could pull an exit scam, get hacked, get seized by authorities, etc.
The frozen accounts thing is concerning. I wonder the reason?
There can be many reasons, generally financial institutions are allowed to freeze assets if they deem your account suspicious - there's no strict rules here.
And they can freeze your account for whatever reason they deem is appropriate. Even if sometimes you are not breaking any condition from their ToS. They can always say your account is suspicious, they have the privilege to label your account as such. So clearly, having your coins in your cold storage is way way better than storing it to centralized platforms.