What happened to the humans in "Cars" (that is if they ever existed)?

Question

In every Disney "Cars" movies there is always some kind of sign of human life. However the terrains don't support the fact of humans never existing:

• Human language; writing and signs on billboards vary in languages and surely languages such as English and Japanese are too hard for beings with tires instead of hands
• Door handles on buildings
• Pretty sure the US flag is even shown at a point

In fact the biggest proof of human civilization existing is the cars themselves. If the cars evolved by themselves, then it makes no sense for them to have:

• Doors
• Door handles
• Windows
• Car mirrors
• Their personalities/characteristics which seem to stereotype certain types of humans.

I do understand that the film is meant for children who don't pick up on things like this but I am just curious. I am not looking for theories.

Answer 1

There is no public canonical explanation.

However...there is a semi-official answer...

The Cars franchise has never offered an official explanation for any of these mysteries. But there is an internal document at Pixar that has never been shared with the public that answers some of these questions, and lays the ground rules for this unusual cinematic universe. It’s called “The World of Cars Owner’s Manual,” and it was written by Jay Ward, the Creative Director of the Cars universe.

^Screenrant

Ward was asked where do the cars come from and gave this response.

"If you think about this, we have autonomous car technology coming in right now. It’s getting to the point where you can sit back in the car and it drives itself. Imagine in the near-future when the cars keep getting smarter and smarter and after one day they just go, 'Why do we need human beings anymore? They’re just slowing us down. It’s just extra weight, let’s get rid of them.' But the car takes on the personality of the last person who drove it. Whoa. There you go."

Ward acknowledged that some of them rules are occasionally broken...but

Number one on the list: “You’ll never see the doors open,” Ward said. “Because the brain and the eyes are in there, we don’t want anything falling out of the side.”

The interview continued...

Ward did offer one other possible explanation for the world of Cars.

“[Cars 3 production designer] Jay Shuster did a great drawing a long time ago of a meteor hitting the earth,” Ward said, then mimicking a “Poof!” sound effect. “And all the humans are gone and all of a sudden the cars start rising up and moving around.”