I get a little bit of say on artwork, then the publisher usually ignores me. I trust my editor, am usually fairly amenable to change. I'm not a prima donna. That's where the meat is. I have found that authors are much more approachable than people in other parts of the entertainment industry.
Authors are human beings.
Other media types are not. Writers reach into their own souls and those of their characters. Musicians are in another sphere entirely, somewhere non-musos cannot wander into. Raymond Chandler died before finishing his last Marlowe book. Robert Parker finished it,then wrote another.