It's the cognitive dissonance thing- They mostly DO BELIEVE the Watchtower crap, but it creates a mental discomfort when they think about it in a way that tries to be specific, such as planning for getting old and dying "in this system" or just counting on the end arriving before old age arrives.
My mother accepted that the end did not arrive in 1975 because "we don't know how many years passed from Adam's creation to Eve's creation, therefore YADDA YADDA YADDA." But since it has been so long since then, she has just become a JW with her self-thinking ability on the matter changed in a way that Flipper describes in this thread- her brain neurotransmitters have been singed. "Well, the end is still soon and no matter what explanations Watchtower gives for delay, it's coming."