Gozz,
One way I see it is that these men are often raised in the "truth" and know nothing else. When your parents are JWs, your fleshly bros & sisters are JWs, your grandparents are JWs, your spouse is a born & bred JW, you know no other way of life.
All your life you have been indoctrinated in this mindset. You believe you are doing the will of God. No other religion matches up to yours. YOU have sayings of everlasting life. Men have to listen to YOU to attain salvation. Salvation is through no other group other than Jehovah's Witnesses. So you believe you have the way, the truth, the life.
It may be hard for those who weren't born into the Witnesses to understand how and why these ones don't see things like they really are. Well, sometimes, they don't and can't understand any other way of thinking. Call it brainwashing if you want, but it is definately doctrinated in them, and they will remain this way for as long as they choose to remain "in the Truth".