You can never verify reality from within consensus reality, but yeah our consensus reality has a lot going for it much like provisional science: in the future something may show things to be incorrect about how we believe they work, but for now the models show overwhelming internal consistency.
Whereas hallucinations, whether shared or private, very often end up being very inconsistent and conflicting. Even if two people hallucinating agree on some of it, I don't think you wanna throw away all the incongruencies just because some of it is congruent.
People are quick to believe just about anything that doesn't really conflict strong enough with something they already know. If you experience vague or esoteric things that are just virtually impossible to completely disprove, it may lead people to believe it might be true. Often regardless of how vague or unlikely it is, or if plenty points to it being random or based on things as unreal as imagination.
I also think that believing in wild possibilities can often feel magical to people whereas they think that a scientific or clinical explanation is disillusioning. There are countless wild ideas and theories, many of which have zero credibility. Don't make a hobby out of entertaining too many, because it's a slippery slope and I don't think blurring the lines between imagination and realistic consistencies (empirical things) is really worth a magical feeling. Especially when science is often plenty wild itself.
I'm not saying don't fantasize, or don't believe anything whatsoever that hasn't been proven... but be very picky about it, it should really make *some* sense and not just be interesting.
Not sure of the exact cognitive tricks involved when people report the same hallucination, but it's important that a third party is able to check that one is not retroactively influencing the other or both influence each other. It's also important to note the circumstances because it's inevitable that some of the time, under identical conditions, people experience the same distortion of the same experience, or close. And what didn't match already can synchronize even more as people influence each other via verbal or non-verbal communication.
Einstein said something about two things being infinite, the universe and human stupidity [and he wasn't sure about the universe]... I think that applies not just for general folly but also for our persistent underestimation of how our mind can play tricks on us, exemplified by our denial of placebo.
@Bagseed:
I think what the brain generates exclusively is the hallucinations from sensory deprivation, and to some extent dissociative holes - in other cases sensory signals also play into it, while some people experience distortion because of psychedelics than others, nobody can perceive without processing or filtering. These experiences are relatively special since the brain has to come up with the entire framework itself, instead of having some actual sensory stimuli to use as a framework. Nevertheless, it's not special in the sense that the brain is so incessant about keeping up a stream of consciousness that it will 'happily' dream up entire fantasies than enduring pure void when self-aware.
Since everything in the body is atrophied and broken down when not used, too long of a void may actually 'rewire' (through de-wire) the brain and/or be detrimental?
Hallucinations is a broad term for seeing or hearing considerably more stimuli than there are signals, so much so that they get their own interpretation or notion, frank hallucinations I believe is the term for e.g. seeing people who are not there whatsoever like on deliriants.