The NTSB is underfunded and understaffed. So it investigates accidents using the "party system." That means the NTSB relies on those who may have caused the accident for help in investigating the accident’s cause. Unfortunately, the "party participants" seldom point the NTSB towards evidence in their files that would tend to incriminate them. As a result, NTSB reports go easy
Right after the crash of Flight 3407 at Buffalo, investigators focused on the aircraft’s deicing system. The question, as explained by former CNN reporter and pilot Miles O’Brien, was whether ice had accumulated on the plane’s wings faster than the de-icing system could remove it, leading to an aerodynamic “stall,” or loss of lift.
But as the investigation progressed, it began to look as though, just before the pilot lost control of the aircraft, the nose of the plane pitched up — not down as usually happens when ice overwhelms an aircraft. That raised an almost unthinkable possibility: gross pilot error. When an aircraft gets…
The whole reason the NTSB exists is to learn from accidents and make safety recommendations so that similar accidents won’t happen again.