Humans are curious and contradictory critters.
Government bureaucracies are even more so.
The government has succeeded amazingly well at keeping a number of secrets. But, yeah, there's been lots of leaks on the UFO info stuff. But between the successful ridicule effort and the many successful disinformation efforts leading to ridicule, it hasn't mattered much, so far.
For the fact that humans are curious and contradictory animals is one of the primary reasons one should believe that ufology is constructed from hysteria. Unfortunately, the subject also draws to itself people suffering from schizophrenia and paranoia. Like convincing evidence of Big Foot, Sasquatch, Loch Ness, Chupracabra, with all the people searching for them with sophisticated technology, we likely would have good information. Beyond its inability to continue to hide information for an extended period, the U.S. government has no valid reason to hide proof of UFOs. Nothing could have protected the information from release for over half a century from the widely varying changes in national leadership. And this does not give consideration to all the other world governments who would also be expected to have such information.
I am open to convincing evidence of UFOs. However, blaming the government for covering up knowledge of their existence, is placing a bit more faith than I can muster in the competence of government.
Muleteam1