When a Toyota begins speeding up, due to malfunction, why don't people just turn off the engine?

It happened again today. A guy calls 911, and says his Toyota Prius accellerated out of control, and he can’t stop it.
He stood on the brakes, and began burning them. After 20 miles at top speed, he engaged the parking brake, while a policecar on his front bumper tried to slow him down.
Is there a reason people don’t just turn the key, and shut off the engine? Does the car’s design stop you from doing this?