Wait, I think I have solved the mystery of the cars universe. Humanity went extinct sometime in the industrial revolution when they were invaded by an alien species. This alien species were slug like creatures with incredibly gooey and weak bodies. They observed earth and saw the vehicular technology we were developing. They realized that they could use this technology to augment their own bodies and make them more mobile and stronger. This alien species then proceeded to kill every single human and animal on the planet so that they could be the total apex species. This explains why there aren’t any animals in the cars universe but plenty of plants. After everyone was killed, these aliens started to insert their gooey mushy bodies into the automobiles, and using their superior medical technology found a way to connect their nerve endings to the mechanical parts in the vehicles to control them. After they managed to do this, they started to continue the development of vehicle technology and started to put their bodies into other kinds of vehicles such as boats, trains, and planes. The species also created vehicular animals to replace the ecological niches once destroyed in the initial invasion of the planet. This is why we see multiple kinds of vehicular animals in the cars movies such as lightning beetles and car birds. If the modern cars movies take place in present day, then that means that the cars species has only been on earth for about a century, and if they are able to go from a model t to a fucking electric super race car in only 100 years, then who knows what great technological achievements the cars species will make within the coming millennia.