Another way to see it: A horse (or any animal) is a goddamn nanobot-swarm with a functioning hivemind that is literally beyond human science in many important ways. Unlike a horse:
* Your car (nor even half of them) does not possess a manufacturing bay capable of creating additional cars.
* Your car does not have a robust self-repair system.
* Your car does not detect strain its structure and then rebuild stronger.
* Your car does not synthesize its fuel from a wide variety of potential local resources.
* Your car does not defend itself by hacking and counter-hacking attacks other nanobots, or even just by rust.
* Your car does not manufacture and deploy its own replacement lubricants, cooling fluid, or ground-surface grip/padding material.
* Your car is not designed to survive intermittent immersion in water.
In both a feature-list and raw-computation sense, we've discarded huge amounts in order to get a much much smaller set that we care more about.