Long story short, we keep doing it because it's a highly profitable gambit for all the usual suspects in our military-industrial-thinktanker complex. Yup, all those drone builders and bomb builders and the F-35 builders and professional thinktank bullshitters and the private contractors who profit from our "war on terror" have a vested interest in keeping that war going in perpetuity.
If we ever won that war, they'd be screwed.
So in this most exceptional of democracies, where corporations are people too, my friend, there's a lot of Manhattan penthouses and ocean-front beach houses dependant on keeping hate and war and weapon sales thriving to the max!
And so far, the US political apparatus has been more than happy to keep the gravy train a-chuggin'.