Not counting those that are born and raised within a high control group (for whom this is the only way of living they know), for those who join, all I can say is that it seems there are a lot of people who are raised to be dependent. A lot of parents these days, and even back when I was a kid, do everything for their kids, ask little in return, teach them how to take care of themselves (my husband and all his siblings left home unable to cook, clean or do their own laundry..two stayed at home until they married), shelter them from the types of issues and decisions they need to make, etc.
They are trying to be loving and nuturing, but they fail to teach their children how to be adults. When these kids are adults, and need to get out on their own, they are ill equipped to make any sort of decisions. They need someone to guide them and religion provides one of the more constructive tools (others end up in controlling relationships or as losers).
Add to this the number of people who really have little faith in themselves and their own abilities. Then you have those that need some reason to feel superior to others. These groups have their members convinced that ONLY they are right, hence the smug final message from Holly. She feels she has it right, and we are all wrong. It isn't a matter of agreeing to disagree, she really thinks she's made the better decision.
I stand by the analogy I made in the other thread, being part of a controlling religion is very much like being with an abusive spouse. The victim eventually buys the garbage that they deserve everything bad that happens, and that if they'd only be "a little bit better" they'd get all the good things and be happy.