The short answer is that military and the intelligence community lied to the politicians and bureaucrats. That is,
they told the politicians and the bureaucrats what they wanted to hear. I’ve observed over my almost 30 years an institutional preference for optimism, even when it isn’t justified. No one seems to advance their career by arguing that a course of action should be avoided. So the default is Pernicious Optimism.
For example - and this is a real world example - the State Dept/DoD wanted 350 dudes graduating basic army training every month. The story goes like this:
Maj: Capt, I need to brief the Boss tonight so he can brief the Commander so he can brief the JTF Commander [and so on]. How many recruits did we get through training last month?
Capt: Sir, 150 graduated.
Maj: What? We're supposed to graduate 350.
Capt: Yes, sir, but only 150 graduated.
Maj. Well, I can't go to the Boss with that. Check off the box on Powerpoint.
Capt: Yes, sir.
https://twitter.com/user/status/1428074797141876737
Talking point: "There's no military solution in AFG."
Taliban: "We won at the end of the gun."
Talking point: "No one saw a collapse in days."
Taliban: "We have approved a course of action that will have us in Kabul by mid-Aug."
...and yet... here we are. All that professional military education...all that intel...
There most certainly was a NEO war game that was planned out to about 200 pages.
And there was sort of a war game back in 2009. The Americans ran through two scenarios for what would be the last place to leave in case of a rapid collapse: Bagram or Kabul? They leaned towards Bagram because there are too many people in Kabul. So, what do they do? Abandon Bagram.