The glib answer is "most don't", and indeed you will find a lot of writers who have other jobs. Journalists are writers, and the relationship there is fairly clear. PR and marketing materials need writing too.
In short, you make your living by a company renting or commissioning your talents. Musicians had only one other option before recorded music - performing - and even then that wasn't always feasible. Think of classical music and its performance demands.
You joke about live readings, but the ancient poets did just that. They also complained of poverty. People really do suffer for their art.