They pray to it 5 times a day. Their entire culture revolves around it the same way naziism revolved around Hitler.
Why doesn't this make sense to you?
Because martyrdom is the highest place of honor in their idea of paradise. They're not playing with a full deck of cards and you're expecting them to cower and run away if we nuke Mecca.
You can't compare this to Naziism. Hitler didn't promise eternal life. He promised a glorious nation that would rise out of economic ruin and control its own destiny. You have to be living to enjoy that fruit. The Muslims need to be dead to enjoy their "fruit", and they can't just kill themselves for no reason. The Wahabbists (read: the terrorists) believe martyrdom is the fast track to paradise and that it is their duty to die for that at any cost. (They killed Muslims in the WTC, too, so why would they care about the population of Damascus?)