Because they need to keep doing more terrorist attacks, not perpetuate open warfare. If they turn the heat up too much, it turns into open warfare and the U.S. invades Afghanistan. If it stays infrequent and random, the U.S. only does a missile strike.
Put another way, if somebody tries to break into your network (even if they fail) every single day, you'll do something to stop it. If they try only every so often, 2 or 3 times a year, you might notice it and be alarmed a bit, but probably wouldn't do anything to stop it. Most likely because 2 or 3 data points isn't enough pattern to figure out an abatement strategy.
In Iraq the modus operandi was IEDs, dozens a day, so the U.S. poured enormous resources into defeating IEDs (billions and billions of dollars), that resulted in everything from signal jammers to the popularization of geospatial intelligence and social network analysis.
If the method had been infrequent, we would have just been on a heightened sense of alert all the time, but not sure where to resource defeat efforts.
Put another way, if somebody tries to break into your network (even if they fail) every single day, you'll do something to stop it. If they try only every so often, 2 or 3 times a year, you might notice it and be alarmed a bit, but probably wouldn't do anything to stop it. Most likely because 2 or 3 data points isn't enough pattern to figure out an abatement strategy.
In Iraq the modus operandi was IEDs, dozens a day, so the U.S. poured enormous resources into defeating IEDs (billions and billions of dollars), that resulted in everything from signal jammers to the popularization of geospatial intelligence and social network analysis.
If the method had been infrequent, we would have just been on a heightened sense of alert all the time, but not sure where to resource defeat efforts.