There's a simple reason why mass shootings have become routine in America -- Americans own more guns per-capita than people in literally any other country on earth. This 2007 data from the global Small Arms Survey, a group funded by several countries, showed that there were at least 89 guns for every 100 Americans, the highest rate of any country in the world.
But even that may be an undercount. An analysis I did earlier this year using data from the Congressional Research Service and the Bureau of Alcohol, Tobacco and Firearms suggests that there might have been as many as 357 million civilian firearms in circulation in 2013 -- more than one gun for every man, woman and child.
More guns = more gun deaths. This is true whether you're comparing countries or states, research from Harvard has shown. If people have easy access to tools that allow them to easily hurt or kill other people, a certain percentage will use them. Simple as that.
It's worth pointing out that shootings like the one in San Bernardino are just the tip of the iceberg when it comes to the totality of gun violence in America. Over 33,000 people died at the end of a gun in 2013, according to the Centers for Disease Control and Prevention. Roughly two-thirds of those were suicides. That leaves roughly 11,000 gun homicides each year.
Guns don't kill people of their own volition, obviously. But they sure do make it easier. And the sheer number of guns in circulation means that somebody who wants to get a gun to hurt a lot of people typically won't have a hard time doing so.
The corollary to this: small-bore legislative fixes, like increased background checks or assault weapons bans, will do very little to address the total universe of 33,000 gun deaths in the U.S. To do that, you'd need to severely reduce the number of guns in circulation -- like Australia did, for instance.
From both a political and practical standpoint, here in the U.S. that may be nearly impossible to do.