It's free and trivial to backup a password database any number of times. Even storing it with a cloud provider is nearly free, because they are so small. You can even automate verifying that all the backups are accessible and valid. When you create a new account with a new service, your daily backup will pick it up immediately.
With passkeys you have to buy multiple phones, sign each of them into every one of your accounts, then keep them physically distributed (no cloud storage for phones). And to make sure they still work you have to periodically manually go and interact with the phones physically, even the one you stored in a bank vault. You also have to do this if you sign up for a new service.
Not to mention the inevitable services that don't allow multiple passkeys.
You also lose an element of security - if someone steals your phone, they still need your fingerprint/face/PIN/whatever to access all your accounts, and you might even be able to lock or wipe the phone remotely.
With passkeys you have to buy multiple phones, sign each of them into every one of your accounts, then keep them physically distributed (no cloud storage for phones). And to make sure they still work you have to periodically manually go and interact with the phones physically, even the one you stored in a bank vault. You also have to do this if you sign up for a new service.
Not to mention the inevitable services that don't allow multiple passkeys.