Flat Earth sociey is not different than quantum healers or angel coaching, Abrahamic religions or East Asian religions. Only the form is different, function is the same.
People have been creating their own belief systems and groups since the dawn of the species. That's actually one of the main reasons why sapients survived against all other human species and was successful in dominating the earth. While organised religions are very young, they have been functional as law and the order of the land for so long a time, they have become 'legitimate' through politics and culture, because first of all it is profitable and beneficial for the state.
Some time around roughly 200 years ago, when Modern State entered the stage and standardisation began it has become the only functional god on earth and its religion; nationalism. The political mask put on wars and invasions for power and resource, for benefit and profit -which had the same mechanism more or less thousands of years ago- collided with the age of information and mass communication.
The incredible speed of information and mind breaking progress on scientific research and technology; the way the information regarding to this progress overall travels has been too much for people. One minute humanity is about a calender around a year on harvest and climate held by a mother deity, 5 minutes later here comes the magic sky daddy and it is all about fairy tales on how he created the world and the precious humans and how he is going to take them to his bosom if they behave, then 'suddenly' just roughly a couple of hundreds years ago, everything gets started to explain by a system and a method (science) that doesn't apologise, neither comforts, nor offers a personal benefit, which defines them just an ordinary branch in a bunch of monkey family. People do recognise the benefits of ecience and technology, but they have a harsh difficulty to understand and they cannot make it their own, they cannot build a personal relationship with it. Most people are not curious. Most people have familes they need to take care of, whom they constantly worry about and they work their ass off. Mostly people are simple. They do not give a fuck about how nature woks. This is something you either have it or you don't. And it starts as a child.
The world got smaller and smaller and finally turned into a global villlage, it is not a some place that you can 'get lost' and 'hide' any more.Everything is 'in the open'. Everything is for sale. To sell a product you need to convince people that they need it urgently. Imaginary problems function pretty much the same way, not as fast. You also do not need to convince them. They are ready to believe they are in some sort of a situation against them. And the possible set of answers those imaginary problems becomes small religious groups. You can also make money from these inventing problems and offering answers to them. In a time people are actually started to get away from old organised belief systems, you can throw them anything and be sure somebody will buy it.
As far as we know, Sapients are the only human sepcies who adorned themselves, their weapons, shelters, clothes..etc. And that is very very important, because besides its basic function of recognising the fellow members of your clan and the group they belonged to in that clan, it also explains a lot about the species' relationship with itself.
Sapients did NOT survive just because they invented tools. They survived long enough to invent tolls and further, because they evolved to BELIEVE that everything they experience in the physical world was for them; in favour or against their existence; had a nonphysical imaginary relationship with them; RELIGION. Paranoia, worship. Ego. Desire. Immortality. "Everything is about me. Everything is for me. I don't want to die, I don't want to hurt."
That BELIEF mechanism only works if there is a mystery and sense of something so big and unknown, it translates to a fanstasy concept; 'Divinty' in human culture, something bigger than themselves, that humans can 'connect' by just believing in it; recognising its existence. But it is actually lost now. It has been lost for some time. But humans will never give up holding on to it. (The scientific curiosity or 'mystery' need a high environment to occur and it does not work like belief systems. It's not personal, tribal in a primitive way. You cannot include it in your daily life in an emotional connection.)
Flat Earth is the result of the same mechanism. Along with every superstitious, religious belief system. Humans are still creating their own little beleif systems to escape from reality to 'go on'. But the problem is not curshing power of the Church any more, it is the crushing power of the State. "They are lying to us to maintain control. The earth is not a spheroid, it is flat!".
Or think about people who keep 'sending energy' to the universe.
Before mass communication and technology people didn't grow up with an accumulation of knowledge that made the universe their background. they didn't look at the picture of the earth when they were a toddler. Universe, quantum, energy...etc were not set of words from daily vocabulary. Nobody claimed sending some energy to the universe to get something bad. Or demand quantum healing to their chakras. There was just the very ancient concept of Karma but at least they didn't treat it like a 24/7 acessible bank account to send to others. Its function was to lead people to standard behaviour and check themselves according to the norms of the society. Now you can pay money and 'learn' how to 'send' what kind of energy to whom or to the universe.
Anyway, long story short, the progress we went through in the last 100 years or so is too much to digest for people, esp. because we keep overestimating humans. We see ourselves in a giant mirror. Our needs are simple. We are all cave creatures sitting around a fire.
If you give all 'this' to cave developers, if you put them in a world nothing is a msytery and everything is possible, achievable that's what is gonna happen. They will find a way to reject reality to create their own safe fantasy. That's how they survived this far. Evolution sucks. Both ways.