Because the second amendment isn't really taken seriously by anyone, not even the gun nuts. If it were taken seriously, it would become immediately obvious to everyone that it has become horribly dated by the advent of modern weapons technology and it needs to be changed. And then we could have an honest debate about where we ought to draw the line between weapons that people not in the military ought to be allowed to have, because everyone agrees the line needs to be drawn somewhere, even if it's just at WMDs. But no one seems to want to actually have that debate.