When someone knows a great deal about a topic, it is a very common experience to find that the way it is being reported on, and the “common knowledge” on the topic, is wrong. The tendency then is to think, oh well, the reporters messed up on this one thing, but then to go back to trusting that they are reporting reasonably accurately on other topics. We are all limited finite creatures with limited time and we can be extremely well-informed on only a few topics, so none of us will ever know just how many false things are being presented to us. But, it is reasonable and rational to extrapolate from the fact that when you do know a lot about a topic the “common knowledge” is almost always wrong, to the idea that perhaps most of what we are told is incorrect. This includes leaving out things vital for an adequate understanding of events – lying by omission. An extra reason for being skeptical, is that experts on other topics have the same experience. If it were just one person whose special knowledge contradicted “received opinion” it could plausibly be a fluke, but it is not.
Anecdotally, I read and enjoyed a book The Great Conversation: A Historical Introduction to Philosophy. The chapters summarized the thinking of dozens of famous philosophers. I took notes and found it informative. However, when it came to a philosopher who I had written my dissertation about, I found the description utterly unrecognizable. I would never even have known it was supposed to be about him if it were not for his name in the chapter heading. Likewise, because my wife is Serbian, I necessarily was a relative expert on the Yugoslav Civil War. I discovered that nearly all the reporting on the topic on US TV and newspapers was hopelessly either just completely wrong or one-sided. I could prove it was wrong because, for instance, I had documentary videoed evidence available on YouTube proving it unambiguously. Just to watch the video was an hour and a half investment of time. So, millions of people were being deceived. One last example is from Solzhenitsyn’s book Gulag Archipelago about Stalin’s prison/death camps for political prisoners. People would be sent there for being “counter-revolutionaries” or “an enemy of the people.” The person being sent there would know that the charges were false, but would assume that a mistake had been made in their case but that the other prisoners were all guilty as charged. In fact, almost none were “guilty.”