Here is a justification:

I would say that generally selling art is not the goal of artists. Yes, they want to be able to support themselves through making their art, but generally speaking their goals are likely some kind of combination of obsession with their craft and a desire to make it into the art canon and attain mass exposure via museum exhibitions/acquisitions, inclusion in biennales, inclusion in art history/theory texts, etc. Working with a good gallery helps to promote the (artistic) career of the artist and establishes their art as culturally significant.