Edit: My original answer was unclear and too brief for someone.
A clearer answer is that outsider artists, as generally identified by profound social marginalization, typically resulting from mental illnesses, are statistically almost never able to support themselves on art-making.
Programs exist for the mentally ill, yet it would be a hard case to make that these are sufficient for creating quality of life such that everyone suffering in that category, and who would rather be making art as a primary occupation, can support themselves and live independently (provided that is a goal) on art making income.
But like many causes, money for this cause is harder to come by than it could be in the wealthiest nation in the world.
Extracting money, not currently available, from the least creative of workers to support creative work and fulfilling lives for "outsider artists" and all others on the margins of the modern economy seems to me a kind of fair play.