It's possible to understand why the military would cover up UFO sightings when the consequences of releasing all of the information are considered. The mentally unstable, the religious power elites, hostile nations, and sworn secrecy provide good reasons for not revealing anything about anything.
There are more than enough mentally unstable people who, no matter what would be released, would find a way to translate the revelations into a bizarre fantasy that centers around them, as the stars in their own show. As it it, the false nature of their fantasies is exposed when they describe abductions and events that use technology that is limited to what we know. Why on earth (literally) would a society that is technologically advanced to the point where space travel is possible need to use something as backward and archaic as anal probes? Even as comparatively backward as we are, we are close to having MRI, CAT, PET and other technology to examine the body, including the digestive tract.
The religious power elites would be very unhappy with anything that challenges the veracity of their mythological meal tickets. If we think of the monetary damage, alone, to the established business of religion that is based on pay to pray, every major entity, from the Bank of Rome to a hundred thousand storefront churches would be calling for the information to be put back in the box! When technology trumps even a small part of mythology, the shamans, religious leaders, and witch doctors are automatically against people knowing about it. Nobody seems to get it that all of the technology produced so far has failed to explain the unexplainable, so religions are safe, even if the truth about UFO's is revealed.
Hostile nations: We might have built the UFO's, ourselves. We just might have technology that operates in ways that are far in advance of any publicly known air and space vehicles, when it comes to appearance, maneuverability, and speed. Why would we blab about it and let hostile nations have an opportunity to reverse engineer the technology for their own purposes? We see what happened when well meaning, but stupid scientists (and scientists have a remarkable capacity for stupidity in areas that are not science) gave away the secrets to nuclear weaponry.
Generally: Maybe our governments have had a longstanding contact and interaction with advanced space travelers, and just do not intend to talk about it. Maybe the advanced space travelers do not want anyone to talk about it. Like the first rule of "Fight Club", the individuals involved in Area 51and other mysterious, top secret facilities invoke that rule and do not talk about it. It is telling that not one of the thousands who work there, or who have worked there have never leaked any information of any merit. The only thing that has been discussed by any military individuals is the sighting of UFO's. Never has an explanation for the object been available, been given, or been leaked. If there were any explanations, something would have been leaked by now.
Finally, with the ability of people to videotape virtually anything these days, any UFO sightings can easily be recorded and broadcast to the world within minutes. And anything that is an unidentified flying object is a UFO. The term means that we don't know what it is, and it is flying. There are also USO, or undersea sailing objects that no one has gotten equally excited about. But none of this means that aliens are about to descend upon us or that the military is hiding anything.