Because they aren’t terrorist attacks. Terrorist, despite increasingly common usage, doesn’t just mean ‘really bad person.’ It implies the use of violence to effect political results through terror or, on occasion, direct destruction of something/someone hated. This is why McVeigh is called a terrorist, for instance.
Killing spree, however, is a reasonable name for it, and I have heard it used, though mostly for cases where it is a prolonged series of murderous incidents. A ‘mass murder’ as committed by ‘mass murderers’ seems the best solution for both precision and suitable affect.
