The whole point of Keybase is we don't have to trust Keybase (as a company). The proofs have to be independently verifiable, and an oAuth login wouldn't be.
I definitely get that motivation. Doesn't change the fact that people who might want to use Keybase are more likely to be privacy-minded, and therefore less likely to have public Facebook, Twitter accounts (like myself).
I'd personally prefer maybe a badge which was showed either publicly or privately verified, and then people can make their own judgement, but I appreciate there's a loss in simplicity then.
The whole point of Keybase is we don't have to trust Keybase (as a company). The proofs have to be independently verifiable, and an oAuth login wouldn't be.