They have numerous government sponsored studies, such as project Blue Book, which can give the impression that they too have been puzzled but unable to come up with any firm answers. Beyond the studies that are known to the public, there are many that have been prepared but are not yet released. These are held in abeyance, and updated periodically to stay in step with the level of awareness in the public, and would be released should the government get pressed to explain the increasing evidence that the Earth is being visited. They are concerned that the whole story about MJ12 and past government involvement with aliens, much of it shameful and even unlawful, will unravel and come out. In fact, the worse the past behavior, the more likely that the cover-up will continue.
Decades pass, and at times half a century, before the truth is revealed.
The worse the lapse, the longer the cover-up before revelation. They are not in control of the visitations and sightings ongoing, and this lack of control is out of keeping with the posture they take in front of the tax payer. All manner of results are feared, from tax payer rebellion to a collapse on Wall Street. Since these issues can be endlessly debated, the debate is never-ending. Thus, there is no point where a decision has been reached and they move forward to a press release of some sort.