Wanna blame someone? Look around. HN is chock full of exactly the kind of people who cause this outcome (albeit many of them are too young to bear any responsibility). Upper middle class types, the kind of people who vote, write the occasional blog, people who's voices are heard politically and who generally will not tolerate violence without some pretext of necessity. They're why we got stuck in Afghanistan. Lower class voters would have mostly been fine with "bomb them back to the stone age and go home". We had to spend 20yr pretending we were actually going to fix things so that educated voters could feel good about themselves. It's like the nation state equivalent of not wanting to sell a project car you'll inevitably never get around to and then finally selling it because you're moving and the prospect of incurring a bunch of cost hanging onto it finally forces you to deal with reality.
But nobody knew what they wanted Afghanistan to be and allowed themselves to be engaged in a war of attrition.
We did have a goal. The building of the nation of Afghanistan into a functioning liberal democracy. Of course Afghanistan barely qualifies as a nation and many (most?) have no desire to live in anything resembling a Western nation-state. Short of a truly brutal, imperialist takeover or the installing of a strongman puppet to do the dirty work for the next 50 years, there's no real chance of transitioning the peoples and their cultures over to our way of things. Americans likely have no stomach for such things, and the state of realpolitik is such that this would be truly untenable on the world stage.
Come on. You can't build a functioning liberal democracy in a place like that while doing zilch to promote even the basics of civil society and liberal norms. (And no, elections and ballot boxes are quite irrelevant; they're simply a red herring. Electoral processes have zero legitimacy to begin with in a non-liberal society.) The whole idea is quite simply preposterous.
Honestly, this should be enough to soft-ban anyone who was in the US Congress in the last 20 years from running again. It is quite hard to describe the scale of this disaster without seeming wild-eyed.