Is it hot enough to "fry an egg"?
Then it's also hot enough to fry a dog's brain.
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 and kill within minutes.
This is not to mention the other reasons you wouldn't leave an animal unattended in a car - they can potentially press buttons, pull levers, potentially set the car rolling or moving, lock the car on you, etc.
But let's just chalk it all up to common sense.