He can't really even be trying to sing on those late '70s and early '80s live alubms.
Listen to side 3 of Love You Live, where he's playing blues songs in a small club. His live vocals there are as good as they ever are. I guess the venue/material brought out something different in him. The contrast between that and the awful performances on the rest of the album, at least in the case of the worst examples, can't be chalked up to a lack of practice.