You can do twenty mixes of something and still never get it right. Their credits are getting bigger and bigger and some records sell better because a certain producer has their name on it. I don't think their work has changed much. Maybe the world is becoming a bit more receptive to or aware of what they're doing. I know I always look at the producer's name on the back of a record before buying it.
Producers do great things with other people's music.
It's like art directors or production persons who don't design anything themselves. If I were a great producer, I'd be out there making my own music. Yes, I guess I am. Not a great producer, but at least I'm producing my own music. I could be completely wrong about the role producers have though, they could all be closet rock stars.