For much the same reason that stars twinkle, except that there are enough light rays coming from the object that no matter how much they scatter, some of them will always be reaching your eyes.
So there’s no actual twinkling.
When you look down the road on a hot day, you may see shimmering “lines of heat” or “heat waves,” and a distant car will appear wavy. What you’re seeing is the effects of light refraction: the bending of light rays when they leave one transparent medium and enter another.
In this case, the light rays from the car you’re looking at are passing through various regions of air on their way to your eyes, air of different temperatures and different light-bending abilities, depending on just how hot each section of the road happens to be.
A light ray coming at you from one part of the car may be traversing a different combination of air temperatures, and hence may be bent by a different amount, than the light from some other part of the car. And that looks to you as if the car itself is bent.
But why does the distorted image keep wavering? Because the rising hot air and other air circulations keep changing the patterns of air temperatures through which the light is traveling. If the consequent amount of ray-bending keeps changing, so does your image of the car.
