Believe it or not, these people aren't doing this to become famous. So getting media coverage isn't the goal.

They do it because they feel downtrodden, bullied, and otherwise being left out and left behind by the world. They want, even if only for a few minutes, to feel powerful and to have people "respect" (or, really, fear) them.

They also feel terribly alone in the world. They aren't interested in being famous, but the coverage of each shooting shows people who think like this that they aren't alone, and that they can do it.

Each mass shooting makes it easier for the next one basically because it is creating a mob mentality in virtual space. People are doing things they otherwise would never consider just because they are seeing more and more other people do those things.