"Who else could it be? is the big reason.
The entire premise of a FDS having been appointed to micromanage JWs' lives is never questioned.On a smaller scale, JWs "know" the GB has "the truth" by constant repetition and by the "big" doctrines that never change:
-- Name "Jehovah"
-- Never-ending witnessing
-- End just around the corner (despite constant new light on when)
-- No Trinity
-- No hellfire
-- Neutrality