why isn't there an OAuth system that is purely GPG keys based. For example I could sign some custom message from the server using only my keys, without ever hav

why isn't there an OAuth system that is purely GPG keys based. For example I could sign some custom message from the server using only my keys, without ever having to deal with emails. This way sign-up is as seamless as login. Is there anything like this I can use? Are websites not doing this because of spam and other issues?