In the X-Files universe, or the Star Trek Original series universe, or the Picard babbleverse, or the Hitchhikers’ Guide to the Galaxy universe, or the Doctor Who universe (in which of many situations), or the Men In Black universe, or the E.T. universe, or the Close Encounters universe, or the Starman universe, or the Cocoon universe, or the Mork and Mindy universe . . . ?
Point being, since none of us AFAIK are in a position to know what “the US government” knows about UFOs, it’s impossible to accurately answer “why?” they might think to hide what they know.
We could make intelligent guesses based on postulates about what they know, but those will tend to be contradictory. For example:
* Maybe they think there are strategic advantages to not revealing what they know.
* Maybe they think people will start doing undesirable (for the oligarchy) things, if they know.
* Maybe the oligarchy manipulates them into saying nothing, for their own reasons.
* Maybe they are in an advantageous agreement with space aliens, and don’t want to share.
* Maybe they are convinced or mind-controlled by aliens, who don’t want others to know.