First, the policy of discouraging discovery can be pinned on dead guys...........policymakers who decided this decades (up to a century or more) ago. Those who finally disclose end up heroes.
Secondly, yes, there are piles of issues that government keeps close to its vest. This is both wise and expected, and they can defend those decisions, even if you or I as individuals disagree strongly.
Thirdly, yes, disclosure itself is a can of worms that supports continued silence, especially if they think they have morality on their side...........or can make a reasonable stretch toward that defense.
Finally, I have actually come to believe that ignorance of these creatures by the vast majority of people IS the wise policy. Do you want to destroy something? Let everybody know it exists. People will screw it up pronto. Guaranteed.