Why would a key need to be "hardware-contained" to be difficult to phish? My SSH private key is unphishable and it's right there in a file. It's unphishable because I know there's never ever a reason to send it to someone - in a scenario where that would be needed, I'd generate a new key just for that situation.

Desktop operating systems don't have very good separation between programs. If some malware gets access, it will be looking for e.g. ssh keys and using a keylogger to get the passphrase if you encrypted your ssh keys. Tpm protected passkeys are much better protected