Because it's really, and I mean REALLY hard to teach the computer to create stylised illustrations from a 3d scene. Even harder, to actually animate the scene g

Because it's really, and I mean REALLY hard to teach the computer to create stylised illustrations from a 3d scene. Even harder, to actually animate the scene geometry in a way that corresponds to what a human animator would draw. Think to one of the core principles of animation - squash and stretch. Every character is more or less a soft body that deforms in a volume-preserving way to add emphasis to movement and intent and that's a really important artistic element of animation. For example, just watch an old Tom&Jerry short and try to imagine how you'd represent these characters as 3d geometry. How would the animator move and deform them? How do you do the effect where Tom is hit with a shovel in the face and turns into a table? With hand-draw animation (either digital or on paper) you are just drawing the final result, but with 3d animation you have to describe the actual physical geometry in the scene.