Don't underestimate the power of indoctrination.
Nearly four years elapsed between the time I began to have serious doubts and when I finally reached a point of no return. The mere possibility that my doubts could turn out to be unfounded was paralyzing at times. I feared making a decision that would foreclose the possibility of my ever going back.
A number of my still-in contemporaries (now mostly in their 30s) have been leading double lives since adolescence yet they still go through the charade. One of them is even an elder now. They never learned to question authority and will probably never leave.
It being "the truth" is a self-evident thing for them. It's true because it's the truth. That's just the way it is. Failed promises, changed doctrine...none of that will change the fact that it's the truth and why would they ever leave the truth?