That's mostly a thoroughly erroneous assertion. Those of us who've worked in TOP SECRET CRYPTO communications and higher roles and classifications COULD cite a list of things that are STILL not only not widely known--not even rumored.
Keeping an intellectual secret is far easier than keeping a physical secret. An intellectual secret is just in your head and on some documents in a safe or a safe room.
A physical secret requires somehow keeping all the laborers sworn to secrecy about what they have built. The Manhattan Project, the underground presidential bunker in Colorado, Area 51, the new "secret" base in Utah, the various "secret" bases around the country for spy sattelite communications, the "secret" anti-aircraft missle batteries at the White House, etc. These are all examples of poorly kept physical secrets. Anyone who wants to know a bit about them can, because its impossible to keep the mouths of thousands of laborers shut.
In some respects about some topics . . . you just happen to be wrong.
In terms of the UFO stuff, ridicule, disinformation, threats etc. have worked very well to neutralize what leaks have occurred.
But there are nuts and bolts things that there have not even been rumors surface about. It depends on how big a project is and how important it is to keep the secret.
There are technologies to monitor and track what every individual involved in a project does and says. Some think that the ET's have had a major hand in our acquiring such technologies and even in helping us manage them to insure secrecy.
Folks who enter such black project never get out. Never. Not only are their lives forfeit--whatever families they have--their lives are forfeit as well.
As the rancher in Roswell on who's land the saucer crashed noted . . . after being kept nude in jail for several days . . . they threatened to kill him and every member of his family if he talked.
That's pretty convincing for a lot of folks.
Would it fail to convince you?