A good portion of today's music is just mixed/layered in a studio setting. Groups like the Platters, Statlers and Eagles could actually harmonize. When you go to a live show nowadays, the difference becomes very clear. My 2 cents is that in the past, music had to market the voice first. Today, they market the person first and just 'fix' the voice in studio. A guy like Roy Orbison (one of the best voices ever in my opinion) wouldn't even get a contract today (not handsome/marketable enough).
Then factor in the ego centered me world we live in today....well, a group of 4-5 people working together and sharing the fame is pretty rare (one direction or 5th harmony maybe???)