Often our most firmly held beliefs come from our social groups, not our own experience. For most of human history, being kicked out of a group would mean death alone in the wilderness. So we are predominately preoccupied with identifying how our beliefs will be interpreted by others, rather than examining if they’re true. In fact the more the belief differs from our model of reality, the more important it becomes to group cohesion, because it becomes a costly signifier that you’re “one of us”. We likely wouldn’t believe these things in isolation, but as social animals they prove useful in one of a number of ways.
Adopting group beliefs help us blend in as one of the crowd, so as to not attract too much unwanted attention from those in charge. As Voltaire says, "It is dangerous to be right in matters on which the established authorities are wrong”. An alternate strategy is to use similarity of beliefs as a way to demonstrate loyalty with those in power. Flattery works at the highest level, and it’s hard to climb the ranks without showing you’re on side. The exact opposite – professing beliefs that run counter to authority – is a high-risk-high-reward approach, which makes you a target in the short term, in favor of long term success if your band of rebels manages to topple the current regime. Adopting really fringe or contrarian beliefs can be a form of Aposematism – a signal that you’re not worth attacking – which is likely the mechanism through which conspiracy theories persist.