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.


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.