It's done on frames because that's how renderers work. You build a description of a single image and the renderer renders it. The renderer will generate threads

It's done on frames because that's how renderers work. You build a description of a single image and the renderer renders it. The renderer will generate threads which will all render a 'bucket', which is just a square block of pixels in the image. It's possible to parallelise this across several processes on different machines, but this is obviously less efficient because you'll have to do all the render startup tasks (reading resource files, building acceleration structures) multiple times.