Well, how did you yourself get 'unbrainwashed'? Why did you believe it in the first place?
Could someone else have snapped you out of it? Would you have reacted defensively to that?
I'm sure one can't give any one reason for why people believe in them but these might play a role:
- The 'excitement' or stimulation provided by these ideas
- It's just another form of escapism, like drugs. Finding relief from life's pressures.
- It may give one a sense of purpose by attaching oneself to ideas ascribed to by a 'select few'
- The need for being special, or to stand out from the crowd.
- The 'messiah' effect: here are the answers to my life's problems.
- Paranoia that isn't tempered by common sense or the use of rational thought.
- Mental derangement; a certain % of the population has got to have a few screws lose, just the law of nature.
Those are the few I could think of