Theory 1: There was a WWII
The Cars-verse includes a WWII-era Jeep named Sarge who remembers events like the Battle of the Bulge, and in the film Planes (made by Disney, not Pixar) there is a WWII flashback where the plane Skipper remembers losing his entire squadron. If planes had a WWII, then it only makes sense the Cars were an ally, right?
Theory 2: There's religion in the Cars world
In Cars 2, there was a Pope Car, meaning Catholicism exists within their world. How many other religions do you think are practiced in the Cars universe?
Theory 3: There are criminals in the Cars universe
In the second film, the characters have to go through a TSA-equivalent checkpoint. Meaning there must be concern around suspicious activity. Seems suspect....
Theory 4: Cars aren’t actually cars, they’re highly evolved bugs
First of all, we know they’re organic creatures because of their basic needs, like oxygen. In Cars 2, Finn McMissle uses an underwater respirator. The Cars also ingest real food, like pistachio ice cream, sushi and snack food, and get the hiccups revealing that they have a diaphragm in addition to a stomach - just like living creatures, they have internal organs! Second off, they show they can fly - the VW Beetle is able to develop wings just like a beetle bug and Mater can fly. These wings are not made of metal like other car parts, but Chitin, the real material of an insect’s wings. And just like the body of an insect, the Cars have a hard exterior or exoskeleton that’s made up of multiple plates and is connected to the internal muscles.
Theory 5: the Cars are the future of humans
We know that these Cars had to have existed with humans at one point because of their features that are catered to human needs, such as the door handles and mirrors. Plus, the Cars use the same written language as we do - if we had never been a part of their world, they would likely have their own form of scratch made from tire wheels. So, what happened to humans?
Well, one theorist believes humans and cars evolved to become one. In the age of autonomous cars, humans become so dependent on their cars that they begin to spend all their time in the car - they feed off the same fuel, are connected via mechanical linkages to steer the car and manipulate the limbs, and even have a human waste system inside the car.
That’s why each of the cars has a fundamental human personality, as they’ve taken on the personality of the human inside, from their culture to language to intelligence. That’s why doors don’t open (the humans inside can no longer survive exposure to the real world) and why Cars can usefirearmss against one another - you can kill the Car by killing the human inside. And of course, this theory explains the location of the eyes on the windshield - a massive magnifying lens connected to the human’s own face.
Image sources in order of appearance: