Is it hot enough to "fry an egg"?
Then it's also hot enough to fry a dog's brain.
- U.S. Public Service Announcement from 1987
The answer to this question is common sense; you don't leave a child unattended in a car because the interior of a car can heat up intensely within minutes. Dogs in particular cannot sweat, they can only pant; this heat can affect them even more severely than other animals.
This is not to mention the other, less-obvious, reasons you wouldn't leave an animal unattended in a car: they can potentially press buttons, pull levers, set the car rolling or moving, lock the car on you, defecate on your upholstery, urinate on your clutch, etc.
But let's just chalk it all up to common sense.