Because that's what they do in aircraft crash investigations.
Also, it's a matter of storing the evidence and examining them later. There was no valid reason for selling them off as scrap, and the excuse that that happened because there would have been no time to examine them is illogical, either, as the investigation extended to many years.
On 9/11 the aircraft had no need to be put back together, the planes were crashed by terrorists, it is a crime, not an accident. Aircraft are put back together to solve why, what caused the accident. On 9/11 the aircraft were crashed on purpose, there is no need to put Flight 93 back together, we have the black box, it shows the terrorists flew the plane into the ground. We know from passengers, black box, CVR, ATC tapes, who caused the crash.
There is video evidence fire caused the collapse. Steel saved, was unique, or near the impact zones; however damage to the steel made it hard to identify where the steel was from in the WTC towers. Don't need steel saved to know who did 9/11, 19 terrorists committed the crime. The massive crime was done by 19 terrorists, ignoring this fact is not being skeptical, it is ignoring evidence.
Do you know NIST Goals, and with a few samples of steel, able to confirm the WTC towers steel was right, as built. NIST was not out to solve crime, NIST had goals which you ignore. Don't need steel to know who did 9/11. Fire caused the collapse, what is your point; did you see fire, do you understand fire science, and the properties of steel.
Knowing the design of the WTC towers is enough to solve how they collapsed; what is the point. If you want a new investigation, what is the purpose - we already know the tower collapsed after the planes hit due to office fires, the biggest ones in history.
Fires has totaled highrise buildings in the past, nothing new on 9/11 except the fires were not fought in 1, 2, and 7. Fires not fought.
What are you skeptical about? Fire caused the collapse, unless you have other evidence.
19 terrorists took four planes and crashed them, nothing to be skeptical about that unless evidence is ignored.
Ignore the evidence, ignore engineers, that is not being skeptical. Why do we need to put the planes back together? 9/11 was not an accident, NTSB does accidents, FBI does crime. If the FBI thought the steel was needed to solve why 19 terrorists took four planes and murder Americans, they would have saved the steel. It was not needed to solve the crime.
If you don't understand steel fails in fire, and a WTC tower floor fails at 29,000,000 pounds, you are not a skeptic, you lack knowledge of the WTC structure, and the effects of fire, the properties of steel.
When we know a pilot crashed a plane on purpose, we don't have to build the plane - it makes no sense. You are confusing solving an accident cause, vs solving a crime. To solve 9/11 the FBI found who on the planes had motive, identified by last words from crew and passengers about who did what... 9/11 was solved on 9/11 because the terrorists used airline tickets, and left DNA behind in the impact areas, and in the places they were at before 9/11.
What are you skeptical about? 19 terrorists did it, due to evidence, fire caused the collapse, due to evidence, science, engineering etc.
Did you miss the FBI investigation, the biggest in history? Did you tell the FBI they needed to save the steel, and why? What did the FBI say. WDTFBIS
Because that's what they do in aircraft crash investigations.
Yes, for accidents, but not when a pilot crashes on purpose. Crimes and accidents are different.