After all, one major security measure is airlines' ensuring that luggage travels with its passenger, and if a passenger has refused to board a plane, that passenger's luggage is also hoiked off, for fear that it will explode!
A policy that made sense when terrorists didn't want to die. You can deal rationally with the IRA, because suicide wasn't part of the objective. Erect enough obstacles that it's clear they'll die in the process, or end up in gaol for life, and they move on to other exercises, such as negotiating with the British government. It's that scene from Heat ("You see me doin' thrill-seeker liquor store holdups with a "Born to Lose" tattoo on my chest?") writ large.
It's an entirely pointless policy if your opponents are nihilist suicide bombers, because dying in the process isn't just a risk, it's the main appeal. So they die in the process of killing hundreds of people. Great for them: much better than not dying.
A lot of security policies dating from previous terrorist groups rely on our opponents being rational actors. Just as it's harder to defend computer systems against teenagers who assume they won't be caught rather than rational blackmailers who want to make money without going to gaol, it's very hard to defend airports against people for whom dying in a hail of bullets or a burning plane is something to be aspired to.
The Glasgow airport incident was a real insight into this. They were really, really, shit terrorists. Only in the world of shit terrorists is a car full of gas bottles a serious weapon; building thermobaric devices requires huge amounts of research and is still very difficult, because you need to form an aerosol so that there is excess oxidiser available, and only then ignite it. But their main aim was to die as martyrs, and a car fire is as good a way as any to manage that.