The most convincing answer I’ve gotten to that question is that fostering terror is only one of the aims of a terrorist attack. These attacks also function as recruiting, and as fundraising promotions for your terrorist organization. There are what you might cal business considerations, in other words, and those business considerations dictate the kinds of attacks that terrorists want to carry out.
Thank God, randomly shooting one person every week or so does not satisfy the business plan. Terrorists want large, splashy attacks on specific sorts of targets that have high emotional resonance for both the victims and the people on your side who you hope to recruit, or tap for money. This helps explain why Al Qaeda was so obsessed with the Twin Towers, a place that–until they fell–most New Yorkers regarded as a rather ugly landmark containing some so-so office space. To a terrorist group looking for publicity, on the other hand, it had immense symbolic value: the tallest building in America’s biggest city, with the hubristic name of “World Trade Center”.
That’s why we don’t get high-frequency, low-intensity attacks on crowded spaces near some Texas town that no one in Abottabad has ever heard of. When attacks on those places happen, they tend to be the provenance of local lunatics for whom the nearby mall, or primary school, has some immense symbolic emotional importance.