Why are artists poor?

That's the wrong question.

In a state of nature, EVERYONE is poor.

Getting out of poverty means one thing: creating something that other people consider to have value, and then exchanging it for cash.

Heinlein had rules for writers: 1) you must write; 2) you must sell what you have written.

This is true for all artists. It's also true for all doctors, accountants, and software engineers.

If an individual artist is poor, he or she either isn't creating anything of value, or isn't finding the market and selling the work.

Period.