I'm trying to empathise with them... I get why people become radicals of other kinds... but saying no music, no sex, live by the book, behead those who disagree... what part of that makes people think "I want to get into this, this sounds fun"?
The Arab world has valid grievances, which motivated 9-11 for example, but there's more to it than that.
There's a book, engineers of Jihad, that noticed that engineers are statistically more likely to join islamic terrorists orgs than other people.
That could be a reverse-causation: people with jihadist ambitions go and study engineering.
They considered that but the study couldn't find such a causality.