Because every attack in the West is now being perpetrated by people born and bred in that country with the exception of some Belgians in the last Paris attacks. It is too hard now for foreign-born extremists to get into Western countries so organizations like ISIS rely on radicalizing people already there, some even not born to Muslim families. I note that you ignore my point about having the political will to deal with the reasons for radicalization - is that because you want to keep hammering away at immigration and refugees?