Al Qaeda is a Wahabbi group that arose from a custom of the Saudi monarchy. When unemployment rose among young Saudis, who could not be expected to do the ickier manual labor jobs done by Pakistani and Indian emigrants, the monarchy gave them jobs as Religious Police, The Guardians for the Prevention of Vice and Encouragement of Virtue. When they were trained by the local imams into fanatical fundamentalists, they noticed that the Saudi monarchy was itself hypocritical and corrupt, drinking booze, womanizing in Cairo, Beirut and Europe and such. Then some of these religious police volunteered to fight the Russians in Afghanistan, and they because armed and dangerous fanatics.
The US government policy has always been, give the Saudis what they want. The Saudi monarchy wants to stay in power. The US gives them the arms and the surveillance equipment to do this. Without US help the monarchy is pretty much helpless. The Saudi monarchy has two principles: the royals can do anything they want, and they can buy whatever they need to continue doing it.
The Saudi religious police have no way of influencing the monarchy. To the royals, they are simpletons that have been given stupid make-work jobs. The religious police, on the other hand., blame the corruption of the royals on the US, which has served as a procurer for sinful Western ways: whiskey, loose women, immoral TV shows and films, jet aircraft, whatever. Left exclusively to their own devices the best the Saudi royals could do is have fancy tents and palaces, faster camels, better hunting hawks and perhaps a trove of gems.
The religious police that evolved into Al Qaeda see the world outside Arabia as evil and corrupted by wealth. The most ostentatious symbol of this corrupting wealth was the WTC. The White House and the Pentagon were also sources of support for the corrupt Saudi royals.
Just as the Saudi royals see themselves as exceptional beings that are not bound by their own fundamentalist religion, the American oligarchy sees itself as beyond morality as well, supporting the Saudi monarchy and its whims for the benefits bestowed on Big Oil and Big Everything else by trade with the immensely wealthy Saudi monarchy.
There is a reason that 9-11 was not staged by irate Kashmiris, seething Sikhs, destructive Danes or bilious Bhutanese. It was a foreseeable act of payback for the US supporting the Saudi monarchy.