It's because they only expose their minds to one type of mindset for a long period of time. They have basically willingly "brain washed" themselves.
Conspiracies and Cult followings often fulfill a subconscious need. Usually the need to feel important, or to know something that someone else doesn't know. Or to be special. You can often tell which needs the leader or the creator is trying to satisfy, because they repeat it. For example. Things that have been said over the years.
1. I love the poor uneducated - There is a group of people who believe they are just as smart as people who go to college yet they choose not to go to college but feel the need to always prove to someone they are smarter.
2. I know more than.... - This addresses #1. and the empty void of wanting to know something that no one else knows. This also distances willingness to seek other sources of knowledge or to even verify information. We often see this in religions where some will say all your answers are here and they will hold up a book.
3. I can do a crime and I would still have followers - This addresses the mindset of those who are willing to look in the other direction when a crime is done.
4. I'm the chosen one - This is a major red flag, because it's the same thing all cult leaders say. In reality if a person has to declare that they are the chosen one then they are probably not the chosen one. "The chosen one" would be something that one could not declare for themselves.
5. Only I can fix the problem - This is another isolation technique to get people to void everyone else but the leader.
6. Believe Me - Again an isolation technique. Don't believe anyone else only believe them.
7. No one is better than me - Another isolation technique
There are more examples - All of these have been used by other cult leaders and mass manipulators through out history. The key is to soak the minds so they do not think on their own. By soaking I mean you submerge it into one perspective and not introduce it to another.
Examples of things that may seem familiar
Lying and Liars
The great masses of the people . . . will more easily fall victims to a great lie than to a small one.
Adolf Hitler: Mein Kampf source:
Adolf Hitler: Quotes
The manipulation of humans is an interesting subject because you see the same pattern over and over again. But the key is to always soak those brains in one mindset.
If you want your shirt to only be yellow, then you must only soak it in Yellow Dye. Allowing another dye to enter will dilute or take over the color of what you want your shirt to be.