It is not what we know. I know it to be fact. USAF and Air National Guard pilots have pursued UFOs, and have filed reports which are never made public, nor can they be made public through Freedom of Information Act applications. I am acquainted personally with an incident in which the Vermont Air National Guard was scrambled to intercept a UFO which had passed over Burlington International Airport.There would have been no scramble if the USAF's true position was that the UFO did not exist, or was a weather related phenomenon.The pilot later talked about it, before he was brought up short, and later had nothing to say at all about it.
The military's position on UFOs is that they have never encoutered a UFO. All such incidents are explainable by alternate means.
So why is the military insisting that the phenomenon does NOT exist? Simple. They do not want ordinary civilians involved in examining the phenomenon,and as to why that is , there are many, many theories and explanations.