It doesn’t seem at all weird to me that paintings have completely different standards than films. Using a ghost painter does seem a little like cheating, if the painting is sold as being the creative and manual work of the named painter. It seems slightly dishonest to advertise it and talk about it as the creative manual work of one person, but create it using other people. I mean, I’m sure it really does happen and that some people don’t care, but still it doesn’t seem clearly and cleanly ethical unless they advertise the paintings as cooperative works and credit the painter.
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.