Secrets are very difficult to keep, but much of modern society is built on institutions that are very good at keeping them: armies, intelligence agencies, and even companies with trade secrets.
So your initial position is much too extreme to be justifiable.
I think the mindspace here is what I was taught. A shared secret is no longer that. That the more people which know, the more impossible it will be.
As humans, we must conceptualize different sides of the problem, differently. As a person attempting to maintain secrecy, the impossible nature must be kept in mind at all times.
This is doubly true with some personality types, which think everything is a game. I've often wondered if some people feel mental pain, when held to their word.
So framing is important.
But from the other side, one can point to exceptions to the rule.
People go to jail, are caught out, fired, relationships break up, governments fall, because conspiracies of even a few people fall apart. It's a regular thing.
You're mentioning some things, of which some were known during the time. The FBI had reports of the 9/11 hijackers, Bill Cosby was known by many but the women were paid off with cash or jobs (until they rightfully went public), and things such as the CIA and the Manhattan project are laced with highly patriotic people, and the death penalty or life in jail, along with people sequestered and watched by immense security.
Secrets are very difficult to keep.
The things you think of were enormously unusual in that they were kept.
They were the exception to the rule.