The normal rule of gunnery is to shoot, and then whatever you happen to hit: call that the target. ;-) With terrorism, whoever you missed is the target. And whoever you hit, is your weapon against that target. But in order to work, it requires the cooperation of the target. If the target does not choose to react fearfully, then the terrorism does not accomplish its objective.
Does the same thing apply to carjacking? Armed robbery?
No. The goal of carjacking is to get a ride; the goal of robbery is to obtain value. Deciding to not fear it, does not deny your adversary his goal.
But terrorism is about persuading the survivors, the technically-not-victims. Nobody ever carjacks in order to get the next car to lock their doors. Nobody commits armed robbery in order to manipulate a third party (movie script counter-example: Die Hard, but the FBI was manipulated as part of a "Briar Patch" strategy, rather than terrorism(*)).
e.g. Not Terrorism: "Your tank factory and its workers are gone. This gains me a numeric advantage in next month's tank battle." Terrorism: "Your tank factory and its workers are gone. Surrender or else I'll wreck more of your expensive factories and kill more of your workers."
(*) Does this happen in real life? What believed acts of terrorism were actually not?