Many arts institutions are thought to be natural monopolies

This is because art being human activities involving the creation of visual-auditory or, in other words, the expression of authors' imaginative or technical skills aimed at expressing the beauty of something that cannot be done by everyone and by anyone. It requires technical skills that not everyone has. In fact, the arts are all about talent and not training, you could train for years and not become an artist, so the few art institutions that accommodate the few talented people become monopolies, and by that, they can raise prices as they wish.