Because murder by gun happens fast and the situation is chaotic.
And yeah, it does happen sometimes that a good Samaritan with a gun will try to help. Those instances do get reported. It happens, but its rare. Because most people don't carry a gun except maybe in Texas and a few other places. And sometimes said people get shot themselves because they are mistaken as the perpetrator.
And yeah people still die in mass shootings even with others armed.
so yeah, it's like maybe we should consider most kinds of guns are a bad idea to have in the hands of civilians.
America is the only nation that has so many mass shootings. We throw up our hands and say its not preventable, and yet we're the only country where like ten of them happen a week. It's a regular daily thing. Meanwhile other countries have mass shootings down to like 1 percent-obviously its preventable. Maybe it's time to learn from other counties when it comes to modern gun control.
And before any nutters say you can't take away our precious constitutional rights for automatic guns that the framers gave us. The constitution was always supposed to be "For the people, by the people" and if we are sick of guns, we can change things.