Why is modern pop music today so terrible?

A few years ago Billy Bob Thornton claimed that there was no great rock and roll produced after 1979.   Maybe he meant by any performers who began their careers after 1979.   I listen to Casey Kasem's top 40 re-runs from the 1970s on Sirius and though there was some mediocre pop music from that era there was also some great music.   I am disappointed in most contemporary music of all popular genres but people like Randy Newman and Paul Simon are still making new and quality music.