I think many join because they love the idea of justice in a crazy unjust world. They've seen/experienced family turmoil, a crazy parent or mental/physical/sexual abuse. Loser parents run roughshod on innocent children - there are gazillions of jealous, envious, controlling, egg/sperm donors, (and that's all they are in my book!), in the world. I think of my childhood friends and out of several there may have been one, adequate, responsible, parent. Thus cults have a wide open market for recruits who want to right the wrongs of life and do better especially before science revealed reality or got through the ignorant skulls of many of us.
Plus years ago many of us were brainwashed that only churches could provide high morals and once many become parents they want their own children to be protected from the terrible world. It's been mind control.