The typical Congressional subcommittee chairman or cable news anchor or syndicated columnist can’t really imagine not being able to afford to take his child to a doctor, or being wrongly convicted of a crime, but he is quite capable of imagining being on a Paris to New York flight that’s blown out of the sky. And while it’s true the risk he faces of suffering this fate are very close to zero, they are not, as they are for a poor person, literally zero.Depressingly, this is probably quite close to the mark (it has been noted before how quickly conservatives get interested in progressive goals the very instant they can be of personal benefit). There are other things to take into account here as well, though. I think this ties into how badly people process incredibly high cost events with incredibly low probability. Whilst the people we're talking about are never going to end up on death row or unable to pay the necessary bills to have a tumor removed, that doesn't explain why they're not getting worked up about things like domestic armed crime, which is most certainly something they're more likely to get hit by than a terrorist attack. Both events, however, will both be metaphorically shoved in the same box, marked "Pretty damn unlikely". And since a terrorist attack is considered as worse than being shot in the street, it's the latter that scares these people.
Of course, that immediately leads us to the question of why people view a terrorist attack as worse than a shooting. That, I suspect, is based in three things. Firstly, in turns of overall damage, an individual successful attack is worse than an individual shooting. Whilst it might seem strange to assume someone who apparently couldn't give two shits that two hundred of his fellow citizens died last week for lack of health-care might be concerned about how many other people die in the event that hypothetically kills them, I think there is some sense to the suggestion that people interpret a higher death count as making it more likely that they themselves will be killed. It's the same thought process that leads to so many people fearing plane crashes more than car crashes. There's a secondary reason behind that, of course, which is that people fear situations where they are powerless far more than they do much more dangerous situations in which they have a modicum of control. You might be more likely to die in your car than in a plane, but in a car you are (or feel you are) directly in charge of your own safety. That seems to count for a great deal.
I may be extrapolating past the point which would sensible, but I would venture a guess that the same thing is true when comparing, say, an armed robbery with 9/11. If you're facing a man with a gun, you might be able to overpower him, or get out of the line of fire, or drive away, or what have you. If you're sat in an office block and a plane crashes into you, you're entirely screwed.
The third and final point is that terrorism is new. Or, at least, it seems new, to the average American citizen. America has been living with the threat of being gunned down in the street for generations. The possibility of dying at the shitty end of a 747 isn't really something people have been thinking about for too long. It's human nature to prioritise threats that are unfamiliar.
So you have the inability to rationally process relative risk, combined with the idea that greater control is preferable to lessened risk, and with the twin fallacies that rare events with high body counts are more dangerous to any given person that much more common events with low body counts, and that a threat you didn't know about until recently is more deadly than the one that's been outside the door since before your parents were born.
So how come people aren't terrified by global warming? It's brand new, at least compared to being shot (and compared to terrorism, for that matter, though that's not an argument liable to gain any traction in America). The potential body count is truly terrifying, we have essentially no individual control, and all the signs say the probability of catastrophe is isn't even the range one can safely pack away as "remote". Why isn't everyone screaming every time the thermometer ticks up another notch?
My wild speculation continues apace. Terrorism is a known, demonstrated threat, which means people fail to grasp the low probabilities involved and thus overestimate the expected loss that a world in which terrorism continues will suffer. Climate change, in contrast, is a threat many see as abstract. Thus they artificially reduce the probabilities involved, so as to underestimate the expected loss to the world (not that most people will think in those terms, of course).
Judging the probability of an event by comparing said event's seriousness and immediate proof of its possibility. It's a fascinating but potentially very costly phenomenon.