Every culture contains different things that can be owned, or activities that can be done legally which lead to deaths of others (even in larger numbers than school shootings). Each country gets to determine which one of those threats it chooses to try to control.
Why are guns legal? I don't trust most people with a cell phone, let alone a gun.
Why is alcohol a legal drug? Because it's culturally accepted, and making it illegal didn't work... despite the large amount of deaths by DUI, assaults, domestic abuse done by those who are intoxicated. Combine it with any other risky activity and you greatly multiply your chance of a fatality.
Why is driving a personal vehicle legal? Look at how many people get killed by vehicle accidents.
Why are samurai swords legal? Does anyone really need a sword?
Why are knives legal? (excluding kitchen, or tool versions like box cutters, utility knives, swiss army knives, Gerbers, Leathermans, etc)? What use do I have of a knife designed to kill people?
Why is sugar and terrible food not only legal and available, but legal to market to CHILDREN? Holy shit that stuff is killing folks left and right.
Why is tobacco legal?
The sad thing is if you try to eliminate or regulate any of these harmful things you'll run right into a wall of people standing up for their rights. You can't use my tax dollars on public transportation! I have a right to drive my car! Even if I like to text while driving, and maybe after drinking! I am pretty cynical about people as a whole and don't really let myself get riled up about their stupidity anymore. Just look at the way our energy consumption is going... we like to look for alternate forms of energy, but it would be political suicide to try to tell people that we should probably eliminate TVs, microwaves, toasters, air conditioning, clothes dryers, etc. You can't take away my right to consume energy! Even if it leads to destroying the earth's ability to support human life. I have my rights!
I like guns. I also wouldn't mind if the US decided to take all private firearms and destroy them. I'll find another hobby. I like knives, and hatchets, and axes (not sure why), and sledgehammers, and Oreos, and scotch, air conditioning, clothes dryers, and a variety of vehicles. But really, I don't need any of those and can live just fine without them.