Maybe because the "body" receives a lot of stress and doesn't look very pretty or peaceful in the end.. The brain will be sending danger signals but you can no longer do anything to respond (unconsciousness)
I asked myself the same thing when I read it. It's a quick and cheap method. In less than 15 seconds you won't feel anything anymore. And if you do things right, the discomfort is minimal (always practicing and researching before)
After that time, the body will "fight" to survive until it shuts down forever. But you will no longer be aware of it.