You mean the bullets just jump out of the box and kill people?
That's because these people go to schools where they know they're not likely to encounter a citizen with a gun. Surely even you can understand that. You never hear about these people going into a police station or a bar where off duty cops hang out or to a shooting range. They go for easy targets.
And if he didn't have a gun, he'd use a machete.
The only way to stop someone hell bent on killing innocent people is for the innocent people to have a weapon and be able to defend themselves. Relying on anything else is sheer stupidity.