Clearly this is because they're being sued, and they want to minimize their losses. Sometimes I wonder what the world would be like if we just said "that sucks" and required a full postmortem instead -- no legal liability, just a thorough public review that everyone could read and learn from. We see this in other fields, like aviation; the NTSB investigates and publishes their finding of facts. The field learns from the mistakes, and as a result it's incredibly safe. (Even if you account for random chance, like MH370 or the whole 787MAX debacle, we're still doing better than we were 10 years ago.)
Instead we just keep making the same mistakes again and again. Everyone pretty much expects to get on a plane and arrive at their destination alive. Everyone also expects their megaproject to be delayed and be finished over budget. When are we going to fix this?