The Pixar Theory

Cars

Meanwhile on Earth, machines are left behind to populate the world and run things, explaining human landmarks and traditions still being prominent in Cars. There are no animals or humans in this version of Earth because they're all gone, but we do know that the planet still has many human influences left.

[Some have noted that the world of Cars can't be after humans left because there's no pollution shown in the movies. If you look carefully at Wall-E, however, the world is never shown during this time, so we don't really know how badly the Earth was polluted. It's possible that the machines sent humans away to curb overpopulation and fix the environment without them, but the world was drained of resources as a result of machines populating the Earth. That would explain why the machines abandoned Earth entirely, leaving only Wall-E behind.]

In Cars 2, the cars go to Europe and Japan, making it plain that this is all taking place on Earth as we know it. So what happened to the cars? We've learned by now that humans are the source of energy for the machines. That's why they never got rid of them.

In Wall-E, they point out that BNL intended to bring the humans back once the planet was clean again, but they failed. The machines on Earth eventually died out, though we don't know how.

What we do know is that there is an energy crisis in Cars 2, with oil being the only way society trudges on despite its dangers. We even learn that the Allinol corporation was using "green energy" as a catalyst for a fuel war in order to turn cars away from alternative energy sources. That "clean" fuel could have been used to wipe out many of the cars, very quickly.

[Someone pointed out that "all in all" means the same thing as "by and large" making the connection between Cars and Wall-E even more substantial.]