According to Theodore Sturgeon, "Ninety percent of everything is crap". I think that's worded a bit too strongly. I would say something more like, the majority of everything is low quality. Of course, there's no objective way to evaluate quality, but I think that's true in most cases, by most standards.
Why is this? I think it's because there are far more ways for a work to be bad than for a work to be good, and it takes skill to reliably produce quality work. It's like a dart board. Most throws won't hit the bullseye, unless you're really good at darts.
Some individuals are particularly skilled, and produce mostly high quality work. Ninety percent of plays may be crap, but not ninety percent of plays written by Shakespeare. And more generally, while the majority of everything is low quality, a carefully selected subset of everything doesn't have to be.
I think this is part of the reason some people perceive recent works as being worse than older ones. When you look at old books, movies, music, etc., you typically find the ones that we still read, watch and listen to today. When you look at recent books, movies, songs, etc., you're more likely to see a representative sample. The latter will be worse, but only because it hasn't been winnowed by time.
A seemingly paradoxical implication of this law is that by increasing output, you increase the amount of both low quality work and high quality work. The relative proportion doesn't have to stay constant, though. In fact, it could move in either direction. For example, the proportion of high quality work can increase by an artist getting more skillful through practice. Or it could decrease by removing a selection barrier that had been in place before.