In particular, there are so many elderly male performers who are determined to sing the words of whatever tune they are playing, regardless of their lack of ability. They croak and strain and are out of tune. Such 'expression' as they attempt is inept. If that's the best they can do, why bother?
I think these vocals help to explain why traditional jazz fails to attract younger audiences.
Some of the elderly gents seem to think that, in order to be qualified to sing, all you need is to be a bit musical on the clarinet or trumpet and also have the words of the song on a music stand in front of you. But there is far more of an art to it than that. It requires a good voice, intelligence and serious practice.