Because the clay is the most versatile material that exists. With it you can make a large or small piece without much difficulty.

The clay is easy to use, easy to add material or scrape to remove volumes, some clays can even be sanded to achieve a near perfect finish for display design, the brown clay has a color pleasing to the eye and has great contrast to see the light and shadow, great to see the High Light in automotive modeling, can be "polished" with water to give a shine to it, finally, is a material that accepts whatever is done in it.

Automakers still use the clay around the world, because there is no better material to view a prototype car before production. As good as the 3D modeling programs are, is no substitute for viewing the physical model, touch and sunlight hitting upon a model outdoors. No matter how well done the render is done on the computer, it is a controlled environment, we have to see the model in a natural environment to see whether what we saw on the monitor really is right and there is no other way to do this without being in a model in clay.

Many things are only made at the model in clay, ideas that seem solved in the mathematical model on the computer have problems in the physical model and that is where the place and time to resolve and fix everything.