Because these nutjobs who are determined to hurt people will just find some other way to maximize the damage. They'll carry more guns so they don't need to reload. They'll carry shotguns, for a wider spray, or build a bomb full of nails.
Bombs and kitchen knives. Blow torches too. There are special campfire style blowtorches that shoot out flames 2-3 feet long, and can be purchased readily at your local camping goods store.
No amount of control is going to stop this stuff. None. You can take every perceivable source of harm away, and they'll still find something to do damage.
Bombs to me are the most logical step if guns are removed. All it takes is the internet to figure out how to make a reasonably harmful bomb. Either that, or simply a gas station and a 5 gallon tank. Should we outlaw gas and portable fuel dispensers?
Where does it end?
IMO, I strongly doubt there is going to be anyway to fully stop any of this. Tighter security at schools will help. Perhaps taking a few instructors at each school, running them through some level of training, and providing them with a locked desk drawer for a defense weapon. But not matter what you do, you can only work to minimize the damage of a nut job.
Are we seriously going to ban motor vehicles when some one loses it and speeds through a crowded parade mauling innocent bystanders?