In the art world, painting and sculpting are known as ‘fine art’ partly because they’re supposed to be singular creative works that aren’t reproducible and that came from an artist, or in other words they’re judged by the purity of the discipline (according to the Wikipedia page on Fine Art). Prints and photography are second-tier because they use machinery to duplicate images, and they go to lengths artificially limiting the number of copies and including certificates of authenticity just to try to sit above commercial art and ads, closer to fine art. Directing a ghost painter isn’t that far of a step away from selling a print as though it’s a unique original. Artists caught doing that would quickly lose any notoriety.
Films inherently take many people to create, and more importantly many people get the credit. The lay public knows that films are created by large teams & production companies. Paintings, on the other hand, are supposed to be a one-person show, the sole painter gets all the credit. Painting is taught as a solo creative endeavor, the public thinks of it as a solo process, and the narrative of the artistry includes technique because it’s a manual solo process.