Since the Cars franchise is Lasster's pet project
Apparently John Lasseter is obsessed with cars, and he had been looking for a story where he could use cars for some time when he got the idea for the first movie. It's the same reason the sequels are made.
So what if he had been obsessed about something else? Horses for instance (even if I doubt the movie would have been called "Horses"), or extraterrestrials, or orchids, stamps or rocks, or birds. That could have turned into an interesting franchise.