We also have plenty of people that goes nuts for no apparent reason, but its just they do not even know how to use or where to find a gun, and any other weapon is simply uneffective and definitely not as cool as an "assault rifle" when planning to cause as much death as possible.
9/11 terrorists used boxcutters to hijack planes and kill thousands. Boston Marathon was hit by a bomb. There was a guy who got mad and drove his pickup truck into a crowd killing tens of people.
There are lots of ways to kill/hurt many people without using an assault riffle.
Oh, and actual assault riffles are already highly regulated under machine gun regulations, so very few mass shootings are done with assault riffles. That is why they had to invent the new category "assault weapon" to ban, which is essentially just "rifle which looks scary".