If you think about it for a little bit, I think you can derive to an answer. Usually GPG keys are part of the Web of Trust and represent a person or a role. It is public-key cryptography, and not a shared secret. Launchpad allows privileged operations based on the GPG key signatures alone over otherwise untrusted protocols (dput package uploads), which thus must uniquely resolve to accounts.
If you think about it for a little bit, I think you can derive to an answer. Usually GPG keys are part of the Web of Trust and represent a person or a role. It is public-key cryptography, and not a shared secret. Launchpad allows privileged operations based on the GPG key signatures alone over otherwise untrusted protocols (dput package uploads), which thus must uniquely resolve to accounts.