Also, ssh keys are not really supposed to be portable across devices like an identity is... YOU (a person) could/should have multiple ssh keys. You have the one your laptop, ipad, phone, work, all bound to an identity. You don't want to use the same key across multiple devices or locations.
PGP ironically, got this right... and has a nice solution for this, where you can have multiple authentication subkeys tied to a single identity, each individually revokable, plus with key transparency when on is added/removed.
A reason SSH proliferation and lack of persistence matter so little is that that traditionally, SSH keys are used for transient session data authentication, encryption, and decryption, not for continued auth/decryption of PERSISTENT data.
In the case of SSH, if a key is lost or compromised, no big deal: create a new set of keys and distribute the public key(s) to system(s) for which you wish to authenticate to. There's also no need to use the same key for different remote systems --- you can use specific keys only for a specific remote system (making an adversary's task of determining what remote systems you connect to, based on public keys used on such systems if obtained, a bit harder).
The use-case for PGP was meant to be encrypting or authenticating (signing) data which would need to be accessible and/or validated from that point forward. If the sender's public key, or receipient's private key, are not available, valid, or uncompromised at some future time, then the data are either unreliable or unavailable.
Using SSH for more durable cryptographic transactions is convenient. But it also changes the use-case and environment around SSH. That will have side-effects.
PGP ironically, got this right... and has a nice solution for this, where you can have multiple authentication subkeys tied to a single identity, each individually revokable, plus with key transparency when on is added/removed.