Because the stated ideology is anti-questioning and pro-trust. If you were a creature who preyed on the innocent you would rationally seek out such a group.
Another way of putting the same point: if such groups take as the cost of entry the sacrifice of reason, by requiring prospective members to subscribe to a patently untrue doctrine (e.g. the word-for-word accuracy of the Torah, the unique divinity of Jesus, our descent from reincarnated Thetans) they filter out any members disposed to use their minds to question authority, leaving only those predisposed to accept what powerful authority figures say blindly. This population is ripe for abuse as it has been stripped of its social immune system, so to speak.
Religious groups that were not authoritarian will tend to become authoritarian as they encounter modern science and all the members who are disposed to question leave.
[This is a response to the recent scandal about the prominent Washington D.C. Rabbi Freundel who video-taped women bathing in a ritual bath without their knowledge.]