Another example is a raytracer. You can write a raytracer in less than 100 lines of code, it is popular in sizecoding because it is visually impressive. So why are commercial 3D engines so complex?

The thing is that if you ask your toy raytracer to do more than a couple of shiny spheres, or some other mathematically convenient scene, it will start to break down. Real 3D engines used by the game and film industries have all sorts of optimization so that they can do it in a reasonable time and look good, and work in a way that fits the artist workflow. This is where the million of lines come from.