That only works if the message is coming from somewhere that can be linked with Keybase. Say for example I sign a message on Voat rather than Reddit. Then what?

That only works if the message is coming from somewhere that can be linked with Keybase. Say for example I sign a message on Voat rather than Reddit. Then what? There exists thousands of sites that can't be paired with Keybase. Pasting a message into /verify is a lot easier for people than installing GnuPG and verifying it themselves. Still faster and more user friendly even if they already have GnuPG installed.