+ ~90% of the world's population are religious (depending on how you look at it) and they mostly grow up in communities wherein everyone is basically the same religion, ergo, they are not generally 'missionary' , 'proselytizing', nor do they feel they have access to 'special information'.
In fact, it's the total opposite: for most of the world's religious, 'religion' is just part of every day, normative culture in most places. It's kind of mundane, actually.
Jehova's Witnesses, referenced in the article, are a totally different ballgame. They are 'default proselytizers' and have a substantially different world view than most folks directly around them.
+ As for why 'Flat Earth' is considerably dumber than Scientology:
'Dianetics' - the Scientology predicate is very close to psychotherapy. Scientology itself is bizarre, but also very interesting 'armchair philosophy'. For example, I think Hubbard's concept of 'tone scale' is brilliant, though it belongs in a blog of ideas, obviously not an institution.
But Scientology is very complex, there's a lot going on, many levels. I can see how it is 'rational' for people to get caught up in it, eve if it's crazy. Scientology leaders are not stupid. There are tons of otherwise very smart people caught up in Scientology ... it's not just full of idiots and crazy people.
But 'Flat Earth' is utterly stupid. It's like saying 'the sky is not blue'. It's the denial of a simple, almost arbitrary fact and it makes no sense at all. I don't think regular people get trapped in 'Flat Earth' conspiracies. Something has to be almost 'wrong' I think to fall for this one.
I think the term 'deprogramming' would be apt for a regular person caught up in a cult.
For 'Flat Earth' - I don't know what one would do with one of those people.