Because in every US mass shooting the suspect is shot dead by the millions of armed Americans?
Oh right the stupity of such a statement is in the name ‘mass shooting’.
Not at all. And, why is that? Because in the US, the great majority of these shooting take place in locations that have been legally designated "gun free zones" - schools, workplaces and an occasional hospital. In none of these places are permitted concealed carry owners allowed to take their firearms, so they don't. Of course, this leaves a target-rich but ENTIRELY defenseless environment for these psychopaths to operate.
What happens EVERY DAY in this country, but is rarely reported here, and it's never reported by your British press, is that people - law abiding gun owners - defend themselves, their property and their neighbors by using their own weapons.
Several months ago, there was a report that had made it all the way over here about a woman in London who brandished a kitchen knife at some hoodlums that were trying to rob her, or cause some kind of mischief on her own property. When the police arrived, it was she who was threatened by the constables for brandishing a weapon, even though it was plainly in self-defense. That's how ass-backwards the UK has become.