Look how inept and stupid most terrorists are.
People talk about the few very successful attacks, people forget how often the terrorists get a boot in their face.
Israel had not had a suicide bombing for years now. That's not because Israelis are uber, it's because terrorists are human failures.
I've read recently a book by Russia's Gen. Troshev. He recounts a story of how his men were dealing with a group of Wahhabis holed up in dug-out positions in a forest. One of his subordinate had the bright idea of getting several dozen dogs and running them towards the Wahhabis. Turned out the terrorists fear dogs on a level of tribal prejudice, and they broke and ran. So they chased them out into the open and brought artillery down on their heads.
Do you think there's been a selection effect at work? That the number of people willing to die for the cause has been winnowed, so they're now scraping the bottom of the barrel?
IIRC, I've seen some articles how the terrorists over there have been resorting to tactics like finding people who are mentally retarded, or extremely socially disaffected to carry out suicide attacks. Or holding some random innocent person's family hostage and forcing them to carry out the attack.
In the relative safety and luxury of America, finding such people may be even harder, and getting them into a group to carry out a coordinated attack nigh impossible unless you import them, and then you run into the problem of finding people competent enough to smuggle or immigrate successfully. Which is a part of why 9/11 took so long, and was in planning preparation for nearly a decade.
I kind of wonder if the cells in America that exist are groups all talking, meeting, planning, with the unspoken elephant in the room that none of them actually want to do it. And the alphabet agencies nab them when the communications and planning rise to the criminal level, and they've exhausted any further intel benefit that further surveillance has.