What is the explanation for why all these governments -- which have changed personnel and leadership through the course of many decades -- would maintain this alleged conspiracy of denial? Politicians are always looking for things to distract people and to give them a cause around which to rally. Seems that it would be in their interest to make the big announcement of alien visitation and to disclose all the evidence. Yet, they continue (supposedly) to maintain this massive cover-up conspiracy. Makes no sense to me at all, especially considering that the same people have not remained in power that entire time. I cannot imagine a convincing explanation for that.
What I find kind of remarkable is that some take "I've never seen anything like it" as evidence that whatever is seen has to be some form of alien vessel. I can imagine hundreds of top secret flight/espionage projects, RPV's, etc., and experiments that would look strange. For example, I can only imagine how the original stealth aircraft may have been perceived, particularly in poor visibility or at night.
For an "explanation" of these events, I'd offer the following:
1) people lie for amusement or to inflate their own sense of self-importance. The fact that someone occupies a supposedly respectable position does not make them immune to that phenomenon, so personal testimonials carry little or no weight for me. Just because someone says it doesn't make it true. Just because 10 people say it doesn't make it true.
2) people are seeing ordinary things, but because of their particular perception, maybe a bad angle or something, it appears somewhat freakish. I can't tell you how many times I've had witnesses describe something honestly, but been horribly, horribly wrong.
3) The military has engaged in some highly secretive experiments with various unusual shaped craft, and rather than reveal that information to the public and destroy secrecy, they keep quiet and let people devise their own fanciful explanations.
All three of those -- and most likely a combination -- make far more sense than assuming that the government -- which is not and has never been monolithic -- would keep alien visitation a secret for more than half a century.