Most old music you hear is good. Most new music you hear is bad. That means old music was generally better, doesn't it? No, it doesn't. It means that music is played when it's good or new, and that if music is old AND bad, nobody wants to hear it, no station plays it, and you probably don't even know it exists.
That's not limited to music, of course. At some point in time, cinemas showed "Plan 9 from Outer Space". That movie is as horrible today as it was then, and because it's so horrible, it isn't shown anymore. This in turn means that many people don't even know it exists, which again means that they don't consider it when thinking about the quality of old movies. And this is even an example of a relatively well known old bad movie. How many bad songs, movies, TV shows, theatre pieces etc do you think there exist that you are glad to never have heard of?