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.
Depends on the region, but mostly it's a combination of imperialism and Saudi funded madrassas, one gives you reactionary ideas and the other makes you believe the ideas are true, also it's used a tool for western imperialism directly or indirectly. For example when you look at ISIS, Abu Bakr Al-Baghdadi was in Abu Gharib at some point, and a lot of the ISIS recruits were from poor parts of the world that had a lot of Saudi "schools" built in them, with the recent attack in Moscow the guys were Tajiks which was formerly a USSR state.
The promise that they'll be rewarded for their efforts in the end goal of a khilafa or in the afterlife... and who told you they don't have sex? you think they kidnap women just to play minecraft with them or something?