I take my cue in one aspect of this from the late Isaac Asimovs' essay "The Rocketing Dutchmen". In this he draws a clear distinction between what he insists on calling 'Flying Saucers', and actual 'Unidentified Flying Objects'. To Asimov this indicates a difference in mindset between those who investigate UFOs as UFOs, and others who are actively looking for evidence of extraterrestrial visitation - 'Flying Saucers'. He held the ideas of alien visitation as being non-proven, and said that if the aliens are there, then they should come down and say hello, or go away and leave us be.
Now I am no hard-line rationalist - "There are more things in Heaven and Earth, etc, etc", but like Mr Holmes I believe it to be a capital mistake to theorise before one has all the data. What we actually know about UFOs could, as my grandfather used to say, be written on the back of a postage stamp with room left over for the Lords' Prayer. I've read a great many accounts of sightings, and while dismissing the obvious charlatans and 'tall tales' such as WW2 'Foo Fighters', we are left with a significant number of unexplained ones which point to a specific and related set of phenomena. But, we do not know what these phenomena are, and I decline to favour any of the multiple possible explanations in view of the lack of solid data.
For the scenarios I presented, I was wearing my 'plain Yorkshireman' hat. Which is to say that many of the ideas presented by UFO apologists violate common sense. If they're advanced enough to get here, they are advanced enough to observe us unseen. If they wish to communicate with us, then appearing before random peope in remote locations is entirely the wrong way to go about it.
I've done my share of reading in the Ancient Alien vein, and it's complete nonsense. It's already been shown that humans had the capacity to build all those monuments with what they had to hand. Also, if you're sending a message to a God who lives in the sky, you need to make sure he can see it, right? Most cutures have or had Sky-Gods, who controlled the weather, and for an agricultural society, the weather is a big deal, so communication with the Sky God was vital.
Governments withhold information for a variety of reasons. Project Blue Book was born out of the Cold War, and its brief included national security. As such, some of its findings were likely classified. The Project was looking less for alien spaceships than it was for Russian spy planes. In any event, the Government, and a great any people, were and are fully aware of what happens when a more advanced culture contacts a less advanced one. Faced with a choice between conquest, colonisation or assimilation, the reaction of the public would be at best unpredictable. Better to keep some of the more convincing stuff under wraps. Don't rock the boat. At a time when people were building fallout shelters in their back yards in case the nukes started flying, the last thing they needed to hear was that advanced aliens might be poking about.
Note also that in these days of leaks, whistle-blowing, hacking and FoI legislation and campaigns, it's becoming increasingly impossible for governments to hide anything.