Because our only strategy for dealing with it, COIN theroy (aka hearts and minds), is complete and utter bollocks and western military planners stick with it despite failing repeatedly and miserably in Vietnam, Afghanistan (twice!), Iraq, etc etc, because they can't admit that an insurgency is not a thing that can be fixed in 2-3 years, and that while blowing up terrorists and militas stops them in the immediate sense, it doesn't stop the next guy from picking up a gun or brick of c4 and picking up where the guy whose now a blood smear left off.
The west's military is not built to occupy nations. They're built to fight other western nations in hypothetical wars, or to blow up tiny nations that displease us and then leave.