It's not obvious to the people who need to be deceived
A lot of what we know we do not know because we have seen it. In fact, most of what we know comes from being told things. When we are told something that does not fit with how we know the world to be, we either investigate it for evidence or dismiss it as false. Most of the time this works. I can tell you that Lincoln Cathedral used to be the tallest building in the world, until it got struck by lightning and rendered shorter. That is true. I can tell you that the terracota army has fewer statues than there are world records held by Usain Bolt. That is, to the best of my knowledge, false.
But neither of these claims will make you believe any others more or less easily. Some claims will. If I tell you that my political opponent is a liar who wants to close down the community centre, that claim makes you lets you dismiss anything they say out of hand. They are a liar after all.
So, what happens when I discredit all my political rivals? Well, you can't listen to them, obviously. But the mainstream news calls me a liar. Me! Clearly, they're in on it too. Wow, there's a lot of liars. Someone's probably behind all this. Say, you ever notice how many Mormons are running things? I wonder if there's something up with that.
So we go from "My rival mayoral candidate is a liar" to "The Mormons are running everything". And someone else might take it further. Now, you'd think something would snap someone out of this. But what? See, any evidence for this is conclusive. Two Mormons on the city council? Proof of conspiracy. And dishonest populism, totalitarian dictatorships, these people tend to have some pretty strong thought terminating clichés. It's all Mormon propaganda/Swedish deception/the lies of the illuminati. Any time anyone says anything that conflicts with your worldview/delusions, say one of these and you don't need to listen. Note that cults also use these tactics. Another one is building identity around a belief, where you become not a lonely guy who doesn't go out much but an Übermensch fighting the conspiracy. This one works very well on people who are going through life changing circumstances.
Another effect of this is that there is a strong emotional cost to listening to anything that would make you less extreme. You can not listen to the enemy without significant psychological pain. Not only would you be surrendering to the villains, but that would mean you were never the Übermensch after all. So for a lot of people the only way to go is further from reality. QAnon is an example of this. Most dictatorships have this to some degree.
So, once you only listen to increasingly radical views, what would do you know? Or at least think you know? You know your enemies are evil, in the same way you know that Australia is real. You haven't seen it yourself, per se, but to say that it isn't true is absurd.
Basically, it is possible to make someone incredibly disconnected from reality, to the point that what seems to be a colossal lie to you seems unremarkable to them.