Terrorists are a simple-to-state example of the problem, but if that doesn't sound convincing, remember that most (or all?) of these fabs are in a region dominated by a superpower that is moving towards dictatorial authority [1], and additionally, that this region is also (one of the many) moving towards a pattern of extreme weather events [2]. There are many possible events that could seriously disrupt such a concentrated industrial base, and only one would have to happen to cause problems.