You can't compartmentalise the planet. It isn't locked inside a room and only certain people get to work on certain bits of it, and they can't talk to anyone else.
But, people's mentalities are easily controlled. This is more than a physical assault.
For example, the threat of the loss of all scholarly articles, blacklisting, and marginalized funding is enough to keep some scientists very quiet about anything one would think should be transparent.
It shouldn't be so hard to understand how the larger secrets are kept secret by dubiety. Another example would be to keep a secret so massive by creating a psychology that punishes challenges to the status quo, and labels some as an idiot.
"If millions of people buy it, why would I think I know better?"
That mentality is controlled; perception, social pressure, professional pressure, and mortal pressure are true tests of how a human will behave - and many humans lie right to your face because 1) you are not their family, 2) they have no obligation to you, and 3) they wouldn't risk the backlash of "hiding" something Joe Schmoe thought he should have known.
Compartmentalization happens on a mass scale, categorically. It is the schizophrenia of society - always keeping
only the host, and the most dominant personality in power.