Actually, the artists are allowed to have monopolies. It's in the constitution.
But your use of the word "monopoly" is unfair because they don't enjoy monopolies in the classical sense of the word. If an author writes a book on the civil war, the author can't stop others from writing a book on the civil war.
A more accurate word is "property" because the law gives the artist much the same rights as a carpenter or a plumber. Just as a team of carpenters can put a lock on the front door of a house that they built, copyright gives the artists the right to control their work. How many homes would carpenters build if any old squatter could just rush in and live for free after the last nail is driven home? I'm happy to give carpenters and other workers what you call "a monopoly" on their work because I want the world to have houses. And I also want the world to have books and that's why I'm happy to give the artists control over their work. It's the ethical thing to do.