The practice of dubbing vocals in films was not always done because the actor was unable to sing for themselves. Sometimes the studio felt that the star's singing voice did not match their on screen persona. Eleanor Powell, one of the greatest film dancers of all time had an accomplished Broadway belt singing voice but in the mid 1930s Alice Faye was the rage so MGM dubbed Powell's vocals with a sultry voice similar to Faye's.
Composer Doris Fisher insisted that Rita Hayworth had a more than adequate singing voice but Harry Cohn insisted that she be dubbed because her own singing voice did not match Hayworth's Love Goddess persona.
Dana Andrews was an operatically trained baritone but no one at Fox bothered to ask him if he could sing when he was cast in the 1945 State Fair and by the time of the film's shooting pre-recordings of his songs had already been done with a vocal double.
In the best of all possible worlds actors with appropriate singing talent would be cast in musical films but this has rarely been the case and the fact that it is not done with the regularity that it was done in the past is because today actors have more clout and if they insist on doing their own singing, no matter how embarrassing the end result or how it hurts the film, that's the way things are in the current film industry or, for that matter almost equally true of today's Broadway musical stage.
As far as auto tune and other sweetening practices go, as Pierce Brosnan proved in Mamma Mia, it only works up to a point and only if the actor in question has at least a minimal natural ability to carry a tune.
"I long-ago realized that this country is a nation of
morons, when it comes to knowledge of anything outside, or beyond, pop culture." Steve Slezak