If you have a PGP key bound to your email: yeehaw@woot.com and you change the email in the key to: haha@yougotserved.com it produces a definitive hash change, while the underlying key material doesn't change.

Contrast that to an ssh key, which has no bound metadata.

And to your point, both systems in isolation are useless. But the first, combined with proofs and a distribution point like keybase, form a complete system. The second however, ALWAYS relies on a trusted party not doing bad things.