They do not "fit in" with the host nation's culture but it can now be argued that they don't have to fit in
The problem is that the values of the muslim ghettos lead to failure in the mainstream of the host nation which leads to frustration and despair and nihilism that canny imams are able to use to turn them into terrorists.
The profile of European born muslim terrorists is of a peace - a youth marked by avoiding integration and education and idolizing gangster figures then in yong adulthood they realize how many opportunities they've wasted and get religion (which they've avoided previously) and the pickings are ripe for radical imams.
If they were in their own culture they'd still be failures (because mainstream muslim cultural leads to failure in the modern world) but they wouldn't have the alienation and frustration.