Presumably because when there’s a shooting and many people die, and people suggest restricting access to guns, pro-gun people say stuff like “if they didn’t have a gun they’d use a knife!”

It’s much harder to kill a lot of people quickly with a knife than a gun.

Or maybe the op thinks someone is going to say “if they had guns on site, they could’ve shot the assailant!” even though that would also be a tragedy.