If terrorism is just about socialising why don't they just go down the pub or whatever -- it's far less likely to get you killed or banged up.

If I may, can't this question be asked another way? If we look at non-terrorist mass murders (curiously, Koreans have set records both in America and worldwide; Woo Bum-kon killed 58 people, and Seung-Hui Cho 32), we notice that they rarely involve extremely elaborate preparations. One has the impression that Seung could've finished all his preparations in just a few hours, even counting the trips to the gun store, videos, letters etc.

Given that it's so easy to kill a large number of people in an immediately doable way, and that methods like just walking around and shooting people are so effective, why do terrorists so rarely actually attack? And why do they carry out such elaborate and relatively ineffective attacks when they actually do? For every effective attack like 9/11, there are multiple attacks which kill only 1 or 2 people or even just the terrorist.

If I may, the social explanation works better. Have you never discussed flipping out or going postal or carrying out a terrorist attack with your friends? Have you noticed that always it is the elaborate and fun-to-discuss attacks you discuss?

No terrorist says to himself, I'm going to follow a boring but effective strategy: I'll enlist, get sniper training, and kill a couple hundred civilians - even though Simo Häyhä killed over 500 Russians under conditions of war and even as the Russians were specifically targeting him and calling in artillery strikes. This kind of strategy would accomplish much more than a regular suicide bombing, but they never do it or any halfway effective strategy. (I refer again to "Why Terrorism doesn't work"; if many terrorists failed to adopt effective strategies, that'd be one thing - but just about all of them?)