Have you ever ridden in a vehicle with rigid tires? Don't. It's painful. That's why you can feel every little weld and junction in a train line, even though you practically can't even see the bumps.

3d printing a tire may have some very niche uses (Mars rover parts for instance), but for general manufacturing, 3d printing is not the way to make a tire. The manufacturers have the process so streamlined and efficient, that there's little room for improvement.