The biggest reason is that most of the Dale Gribbles of the world, just like the local college's Revolutionary Socialist club, are all talk. It takes a lot of commitment to actually carry out a shooting or bombing or whatever, and most people are happy to say "This is really bad. Someone oughta do something, and it's good thing that I'm aware of this unlike the rest of the sheeple" while doing absolutely nothing to effect that agenda.
The people who actually manage to commit these acts are on an entirely different level of nuts.
Pedestrian example: How many gung-ho teenagers talk about joining the military? How many of them actually go through with talking to a recruiter, going to MEPS, and signing on the dotted line? And that's for a cause that is widely accepted by society and confers actual benefits rather than guaranteed violent death or lifelong incarceration in an elaborate concrete box in rural Colorado.
The people who actually manage to commit these acts are on an entirely different level of nuts.
Pedestrian example: How many gung-ho teenagers talk about joining the military? How many of them actually go through with talking to a recruiter, going to MEPS, and signing on the dotted line? And that's for a cause that is widely accepted by society and confers actual benefits rather than guaranteed violent death or lifelong incarceration in an elaborate concrete box in rural Colorado.