Though it's easy to watch the Cars movies and not give their wider world too much thought, upon even a superficial inspection it's tough to parse how its world exists at all.
While the universe within which the films reside contains no humans whatsoever, human influence is still clear throughout, with World War II-era vehicles even causing some fans to consider the possibility that a car version of Hitler must've existed.
Most obviously, though, is the inexplicable fact that all the cars have doors, windows, and mirrors, despite not a single human ever being seen throughout the trilogy.
The vagueness of the world's practicality has caused many to speculate that it may take place in a post-human apocalypse.
Hell, even the series' own creative director, Jay Ward, has stated his belief that humans did once exist in this universe, but were overthrown and eliminated by the sentient vehicles.
It's a fascinating idea yet ultimately too horrifying for Pixar to ever make outwardly canon, so the Cars world is stuck in an awkward state of limbo where it just...doesn't explain itself.