Out of 300 million people, it's bound to happen somewhere. Then the media brings a story to everyone's attention. Meanwhile, around 40,000 die in car wrecks every year and people aren't that alarmed.
Out of 300 million people, your chances of being the victim of something like a mall shooting are very slight. You'll more likely to die crossing the street.
The more people we have, the more times that roulette wheel gets spun and the more chances that somewhere in USA a big shooting will occur. Then, the media brings one incident to everyone's attention.
There might be a rash of copycat incidents for a while.
Is it getting worse, per capita? Worse than in America's past history?
The availability of weapons does play a role. Yes, I am for gun registration at least, but aside from that argument, there might have been practically no mass shootings back in, say 1800.
Back then, it would have taken too long to reload the musket.