UFO people are always demanding that governments disclose everything they know about UFOs and aliens argueing from the position they believe they're real. Well ok, asusming they are real, and the government can confirm that fact, what possible reason would a government have for doing so? Can think of lots of reasons to bury such info, but not one reason to disclose it.
Reasons to conceal such info include:
- "A person is smart. But 'people' are dumb panicy animals and you know it." - "M.I.B."
- Governmental authority would instantly collapse if superior intelligence aliens were revealed as being here. "Why should I pay taxes and follow your laws when there's this superior species who does things another way?"
- Our cultures and socities don't assimilate well relative to others. We have trouble getting along with our own nation's citizens, and tend to marginalize and minimize other country's peoples. Why would we treat those from another planet altogether better?
- The aliens may not be entirely kind, and represent the successful predator of their homeworld. And they're here for reasons we wouldn't like, but have reached a tentative agreement with the US and other nations to do what they're gonna do, but in exchange for our not opposing them openly, must keep themselves secret and their operations discreet. In other words, rather than having them do their thing anyway, but openly, agree to permit them some limited version if they keep their presence secret. (Like the overriding theme in "X-Files")
- The aliens are the basis for all our religions and spiritual ideas. If their presence were confirmed, it'd be seen as a direct challenge to our human ideas about God. If God's just a mortal being from another planet, that realization would be disastrous.
- The aliens are working with one or some governments, but not all. The governments left out may try a Hail Mary type military attack to prevent their feared eventual subjugation by either the aliens themselves, or those with their technology. Imagine if aliens were working with and/or for Iran. Would we just sit and twiddle our thumbs nervously or send in the B-2s?
That's only a partial list. And on the other side, the reasons to disclose such information: