That's the point, though. An SSH key gives authentication, not authorization. Generally a certificate is a key signed by some other mutually trusted authority, which SSH explicitly tried to avoid.

SSH does support certificate based auth, and it’s a great upgrade to grant yourself if you are responsible for a multi human single user system. It grants revocation, short lifetime, and identity metadata for auditing, all with vanilla tooling that doesn’t impose things on the target system.

You can also sign ssh host keys with an ssh ca.

See ssh_config and ssh-keygen man-pages...