The chances of being in such a situation are so incredibly small, that training to deal with it is a wasted effort compared to a host of other stuff — e.g., learning CPR, taking a course in safely driving in wintery conditions, employing elementary fire safety precautions in the house, etc. Even stimulating people to learn to swim if they haven't already saves way more lives!

If you normalize an attack like this to the point that you start training the general populace for its (statistically unlikely) occurrence, you have effectively made terrorism in this form that much more effective.