I think there are at least several reasons and sometimes it can be mainly one reason or a combination of two or more.
I think that some get to the age that they won't allow themselves to consider the possibility that something is wrong because they have too much invested and they're so close to the ends of their lives that they don't like to even consider the possibility that they're just going to die and be gone forever - that it was all just bologna.
And another possibility is that some are just lacking in intelligence and they swallow it all unquestioningly. All the elders in my congregation except one are uneducated, lacking in knowledge, lacking in comprehension, lacking in intelligence, etc. The one exception seems to be level-headed and reasonable and intelligent. He's not formally educated, but he is self-educated (which can be just as good and sometimes better). He is the only one of the elders who senses something wrong. He has confided in me. He said that he has considered stepping aside aside as an elder, but he's worried because his family would be shocked.
Others are just in the routine so deep that they just don't have time to sit back and question and analyze. When a question does arise or something doesn't seem right, they might give it a moment's thought, but then they have to move on to the next thing in their JW busy work so they won't get behind. I think the org by design makes it that way.
I believe that if some of these older ones had more time in life, some would eventually start questioning and doubting. I had a very close JW acquaintance who died in his 90's and was going strong as a reg pio. He was still working hard in the field going door-to-door. He died from surgery complications. I used to argue with him all the time about the fact that the org had said for decades that the end was imminent, but nothing ever happened. He stubbornly defended the org. However, towards the very end of his life somebody said something like "the end is very near", and he responded with something like "Oh, they've been saying that for decades". It was shocking for me to hear him say that.