It's not that the money comes directly through Instagram, it's that the artist grows their audience through Instagram. The benefit is the size of the potential audience pool.

A lot of time has to be spent marketing yourself, unfortunately. To find people who are willing to pay you for your art, you have to be present, you have to be visible. You could be the most talented artist in the world, but if nobody sees your work it won't matter, you'll die in obscurity, penniless.