Simple - his main job was to figure out in what order to put the elements through the optical printer to make the shot work and to make sure all the masks aligned properly, which was a near impossible task in those days and he still managed it. In digital compositing, the same task is laughably easy in comparison. It’s like tearing down the pyramids and rebuilding them using modern cranes and diggers and stuff and saying that they’re the same thing, because they look almost the same only more polished.
Plus optical compositing has a very specific visual aestetic to it, which is lost when the elements are recomped digitally.