The reason I know this alien stuff is nonsense is my experience as a young lad on Air Force bases. My father was career Air Force and I grew up on Air Force bases around the world. One thing I found out is secrets are not very secret.
The men in the Air Force keep secrets but often let it slip to their wives, and their wives talk to each other. You wouldn't believe the things I found out while listening to my mother gossip with other women in a laundry rooms, over coffee, etc.
If aliens fell from the sky on an Army base I have no doubt some men would have told their wives, they would talked to each other, and it would be common knowledge now.
Again there are secrets and then there are secrets. By definition we can only evaluate secrets that have been "leaked". I have heard enough stories about the men that kept secrets to their grave and that there were things that they would not talk about to their dying day even to family.
If such a thing existed as alien visitation I would imagine that the universe of people involved would be very small - making the truth very difficult to penetrate.
I have know several vets of foreign wars and many of them are not very willing to talk about their experience, even to a young vet like me (though granted not a combat vet)
I can see that being involved with aliens would be a very earthshaking experience - even traumatic in some ways I would imagine. that is plenty reason to keep things quiet.
Still the thing that gets me about all these "government can't keep secrets" arguments is that UFO's and Aliens aren't a secret! they have been attested to by dozens if not hundreds of entirely credible people - some holding high offices or government positions. It's just that they are ridiculed out of hand by smarter-than-thou type people who try to act like enlightened intellectuals.