Art has a defined supply and demand, where the supply is high and demand is low (probably due to the fact you can buy copies of famous masterpieces for very cheap).
It isn't about supply and demand, it's about transaction costs.
Suppose you could create a work of art that fifty million people would each value at 25c, so total value of two million dollars. Far more than enough to pay your wages for a year. But if you actually try to charge them, the credit card company wants 30c + 3% per person (consuming more than the entire value) and other payment mechanisms suffer similarly infeasible costs in money or inconvenience or otherwise.
But if we would pay the artist unconditionally and then they create art and give it away, we would get two million dollars worth of art for $12,000 and with no transaction costs.
Meanwhile this allows some of the artists to become famous and then charge prices for their art that exceed the transaction costs, and now they're paying taxes and funding the UBI for the next generation of artists/inventors/experimentalists/etc.
