The only channels worth hearing are jazz, classical, NPR and college stations. And Oldies, but only once in awhile. Oldies, by definition, are already old. While a song being old doesn't necessarily make it bad, these songs were worn out and overplayed the year they were made. It's the ratings, duh.
Radio stations are typically part of a whole chain of other stations owned by a corporate monster.
And they are going to play what the top 20 says is good, regardless of the actual quality of the song. There are lots of stupid people out there boosting those ratings up. And when you listen to the radio, you are supporting the bad music. That's not much time. Radio stations know they must repeat those top 20 songs every couple of hours to try and catch every listener possible.