Because most operators are not aware of how to control a runaway vehicle and are in a state of panic! Most people do not "think" when something like this happens, they have zero situational awareness!
I have been in this exact situation, as a passenger in a vehicle that was in a "stuck throttle" event. The person driving froze up at the wheel, not doing ANYTHING to stop the vehicle. With the car accelerating wildly toward a busy intersection and sensing what was wrong, I simply reached over and turned the key "Off" and the vehicle coasted to a stop, safely. Was I frightened? Yes! However, FEAR and PANIC are two completely different emotions! A person can be afraid and in complete control; a person who is in a state of panic IS NOT in control!
Yes, the driver could turn off the vehicle. However, when one does this, the driver would lose power steering and power brakes. This would create another "situation" for an already "situationally inept" driver. Most drivers do not realize how much power assist contributes to the operation of their vehicle and thus will believe that they "cannot" steer the vehicle because the effort is very high without the engine running. Another problem in turning the key "Off" is that the operator would need to be sure not to turn the key to the "lock" position and locking the steering mechanism, with obvious results.
Yes, the driver could put the car into Neutral, but again the driver needs to be aware enough to think of this AND be certain not to place the vehicle into "Park" or "Reverse", which could cause other issues!
Yes, an aware driver could pull the emergency brake. However, the emergency brake is fairly ineffective in these situations. Regardless, using the E-brake would again require that a driver THINK under pressure!
The common thread here is that a driver needs to be aware of what COULD happen AND be prepared to deal with the situation. There is only one way to be ready for this, that is practice. Silly as it may sound, a driver should take the time to learn how to use the controls of their vehicle in an emergency situation. I have taken my vehicles into empty parking lots and away from traffic and practiced these situations. I have actually drove across the parking lot and turned the engine OFF and drove the car to a stop. I did this just to see what happened! I have stopped the vehicle with the emergency brake, just to see that it works and how long the car takes to stop.
The problem is that most drivers are not capable of performing in emergency situations! American drivers are, for the most part completely unprepared for emergency driving. We do not teach drivers what to do in 99% of emergencies. No one tells new drivers that there are specific things that they can do to maintain control in these situation. Young drivers need to be taught that when the engine quits, that they will lose power steering and brakes BUT they can still steer and stop the vehicle. This is exactly why drivers do not need to be texting, eating, working on a computer or talking on a cell phone! This is also why drivers need to prepare for the unexpected! One thing that ALL drivers can do is to review the owners guide of their vehicle which contains a ton of great emergency information. This is boring and time consuming, but it might also save your life.