The same mental mechanism that causes you to cancel out or turn away from an undesirable thought (like when your brain comes up with a horrible nightmare) - is the exact same way these people govern reality on a daily basis. They've unintentionally shunned reality (what actually is and isn't, in the moment) so many times that it is now completely automatic - perpetual delusion. It's too much effort to think, so they just automatically do, think and believe whatever the herd is doing.