Young Arabs (Egyptians, Syrians, Jordanians, Saudis whatever) were brought up as Muslims.
Whatever accident of where they were born, they still identify as Muslims.
The regime where they live is just the regime. Why should they die for that, as opposed to merely giving way?
Not to mention, wherever they are, a significant part of the populations want ISIS to win.
There is a sliver of hope for Egypt, though. They just tossed out the MB and are hanging its leadership. They wanted democracy, and instead got public crucifications and relatives tossed off buildings. They might be motivated.