What is the likelihood of needing to know how to properly apply a tourniquet, or how to splint a limb/stabilize a neck with whatever is on hand rather than proper medical equipment? But it's taught in health class, just in case things go wrong during your hiking trip.
Why should we have to know to not stand under a tree in a storm, and to not get out of a car in a thunderstorm even though it's metal? But it's in that same health class, just in case lightning starts touching down really, really close.
Why do planes have that oxygen mask training? Air travel, especially domestic, is super safe. But just in case. Also, how much chance is there in a water landing, especially domestic flights? But they talk about the inflatable vest, just in case.
Hell, the Pentagon has a zombie apocalypse response plan.
Pentagon document lays out battle plan against zombies - CNNPolitics - https://www.cnn.com/2014/05/16/politics/pentagon-zombie-apocalypse/index.html
Right now a mass shooting event is happening about once a day in the US, though I'm sure that includes the sad cases where Dad goes nuts and kills himself and the kids to hurt the ex in the worst way possible. Needing to know how to respond to that is training that would also cover a mass knife/machete attack (which have happened), a hostage situation, and being in the same apartment building at nutty Dad.
The outrage over whether or not mass shooting training is needed should instead be focused on common events that no one bothers to prepare you for. Guys, have your doctors been checking for redness or lumps on your chests? Do you have any idea of what to do if you feel what seems like a hard bump in your armpit? 2000 new cases of breast cancer in the US every year are in men. Parents, have your children's doctor ever explained to you what symptoms to look for in case your child is mentally ill? Count off 10 kids around you, one of them statistically will have either depression or anxiety. Why isn't deep water training part of drivers ed class in school? After all, people die nearly every storm by driving through flash flooding because they didn't understand the hidden danger of just a few inches of water flowing over the road?