Find the utopians
The jihadists are modern psychologically as well as demographically because they are self-made men (in traditional societies there are no self-made men). Rather than deferring to custom, many of them have rebelled against local authority figures, rejecting their parents' bourgeois striving and moderate versions of Islam, and their comfortable lives.
They have sought instead some utopian cause to give them an identity and their lives meaning. They find that cause in a brand of Salafism that is not traditional Islam but a modern fantasy version of it, an invented tradition. . . .
In other words, the conflict between the jihadists and the West is a conflict within the modern, globalized world. The extremists are the sort of utopian rebels modern societies have long produced. . . .
They are looking to strike a vague blow against the system and so give their lives (and deaths) shape and meaning.
In short, the Arab world is maintaining its nearly perfect record of absorbing every bad idea coming from the West.
My emphasis. Witness the destructive histories of other modern forms of utopianism: communism and fascism.
End note: While I agree with Brooks, I do not endorse his use of the term jihadist to describe the extremists. Jihad is a term the extremists have hijacked from Islam.