It's not that people are not doing anything as much as figuring out how to prevent these things. I don't think anyone knows. Gun control is an easy default reaction, but back when assault weapons were banned it didn't seem to help. There are some smaller things like magazine capacity that and background checks that may or may not help, but it will mostly just make things a little more difficult. My opinion is that gun control is too late. Mental health issues? Maybe, but how do you fix that? Again, maybe a head start on people showing some danger to themselves or others, but many others will slip through any screening.
What has been done is more reactive than proactive, but it's significant. The training of first responders, teachers, hospital staff, and law enforcement I would suspect has saved many lives. I can think of a few examples where every day citizens being alert to suspicious behaviors has prevented similar events. Maybe added security at airports and other public events has helped. Regarding Jimmy Kimmel's emotional presentation, I can agree, but a lot of the things aimed at international terrorism can be a catch all for domestic acts, too.
Personal opinion, I think toning down hatred and violence is part of the answer. It's in politics and race, but also in popular videos and games and worries about health care. It's in the greed of financial institutions that make dangerous personal information public and angry radio commentators. It seems like it is part of our social fabric. People just seem angry these days.
That's about the best I can think of on a hard problem.