Largely because unless you're part of a very discrete sub-culture you're unlikely to be a target.
More than that, this 'balance' thread isn't about MASS stabbings, just stabbings, the US mass shooter thread has, from what I recall of it, never featured general gangland killings.
the thing that makes mass shootings scary is, while there is a wholly terrible amount of gun crime in the states and murders involving guns, the massive majority of it is one gang fighting another. If you live in a middle class suburb of Denver your chances of being shot in a drive by in a gang turf war are about 0.00000000000001%, where as if you are part of a gang and live in Compton your chances of being shot are dramatically higher. For a random psycho who decides to gun down his office where your wife/husband works, or a nutjob who opens fire in a school, or decides to start shooting from a bell tower on a college campus then the chances of you dying from such a situation in that same suburb in Denver is far higher than normal gang crime.
In other words gang crime scares primarily people on other gangs, their families and those who are stuck living in such areas, 90% of the rest of the US population doesn't fear gang crime and as such mass shooters become one of their day to day fears because gangs don't target random people living in other places but mass shooters target anyone. That's the reason they are reasonably terrorist acts, they drive fear into the masses and that is really their end goal.
This is why a general stabbing thread is stupid and not balanced of equivalent, because the main people who fear knife crime are gang members and those who live around gang culture, everyone else doesn't fear being stabbed at all.