Why does anyone keep anything a secret?
To try and maintain a strategic advantage against potential competitors.
The government is no different than any person or business in that it is in competition with others for scarce resources, and any information your
competitor knows about you could be used against you. So it's never a good idea to let a competitor know everything you know, or conversely,
everything you don't know.
Regardless what they say, various highly specialized divisions of a number of branches of the government involved in national security have an ongoing
interest in UFOs, since these things may in some way be a threat. There is probably a version of MJ-12 somewhere out there, and maybe more than one,
working independently of each other.
It's my opinion, and only my opinion, that the U.S. government has over the past several decades acquired quite a bit of knowledge about UFOs,
however, they still remain extremely ignorant about a very high number of vital, key aspects of the phenomenon and the things themselves.
I don't know to what degree their knowledge or ignorance extends... and that is exactly the point.
The mere suggestion that the U.S. military may be in possession of highly advanced technology derived from alien sources is one more consideration
that other superpowers have to consider when contemplating direct conflict. The last thing they need is for a fleet of U.S. Air Force flying
triangles impervious to counter-attack to zoom in and level their government buildings. Does the U.S. military have this capability, which would only
be used as an absolute last resort? Maybe, maybe not. But it certainly doesn't hurt to have "Brand X" consider it as a possibility.
If the U.S. government and military "came completely clean" about UFOs and what they do or don't know about the things, it would do nothing to help
them maintain the illusion that they have superior knowledge, which is worth quite a bit when someone else is always testing their weaknesses and
vulnerabilities with an eye on exerting some power or threat over them.
So while the Robertson Report might go on about the population's possible social reaction to finding out the "truth" about UFOs (whatever that may
be), a more simple, practical reason for keeping your mouth shut is to not let the other guy know how smart or stupid you are.