Except that's not what actually happens. People working a full time job (or more than one job) don't have the time or energy to also be a part-time artist until they make it. This is especially true for people who have other responsibilities like raising children or caring for elderly relatives. Leaving it up to "the market" only works if a single part time job is enough to live on, and it isn't.
UBI for artists means they can work a little less to make time for making art. Society is essentially paying them to make art. That makes society better. Ireland has decided it's a worthwhile investment.
I'd love to live in a society where everyone is paid to do something they love, to make society better, but doing it for artists to start with is a great first step.