I believe that most dubs remain because of the investment they have made to the "truth".
As soon as men become a MS or an elder they have bought in and can no longer express any doubts felt. Then promotion through full time "service" means an increase in authority over those below and control by those above. Even at CO level every talk and meeting is closely controlled. Only the official line is allowed, the talks are uniform whoever the speaker is.
The higher you rise, the more careful your conformity to the party line has to be. Don't think though that these people though are unaware of the issues and arguments explored in sites like this. They are however intelligent people and must struggle with some of the issues as do others.
What then when they suddenly find a famous white WT envelope on the mat inviting them to become a GB member? Imagine the dilemma, those unanswered questions, the creaking of the structure. Now its your job to hold the whole thing together. You still can't express doubts, you have to bury them deeper than ever or it's hero to zero.
So do they really believe or are they the same as us all but with more to lose?