The fundamental reason that a counterterrorism-focused strategy fails is that it cannot generate good intelligence.
The second reason a counterterrorism-oriented strategy would fail is that, if we tried it, we would likely lose our ability to operate unmanned aircraft where the Taliban and al-Qaeda prefer to hide. Why? If we pulled out, the Afghan government would likely collapse. The secure bases near the mountains of the Afghan-Pakistan border, and thus our ability to operate aircraft from them, would be lost. Our ability to go after Afghan resistance fighters would deteriorate. And the recent momentum we have established in going after Pakistani extremists would be lost.