9/11 didn't just happen for no reason at all.
Well, if by that you mean it wasn't a random attack... yeah. That's true and obviously so. But there was nothing reasonable about it. It was not the action of sane men. You can look at the ideology (both writ large and strictly applied) of the people responsible and try to work out certain things from that, but you can't look for reasons, as such. Because the very word implies, well, reason.
You make the mistake of assuming that everyone is as sociopathic as you.