I think more question doctrine than we might imagine. As stated in comments above, many question, yet suppress their feelings. Some have expressed their personal beliefs that not everything is true, but then fall back into the mantra: "Who else does the preaching as Jesus commands? Who else holds out a hope of everlasting life on a paradise earth? Who else at least TRIES to teach the Bible to other people?" Many simply can't see past all of this, which, to be honest, are very, very large incentives to staying in.
I believe JWs by and large DO think, but I believe they are too tired to address any questions they might have. They're kept too busy with various activities (meetings, family worship night, field service, preparing for each meeting (even at a cursory level), videos on the new broadcasting site on an almost daily basis, daily bible reading, daily text, two new WT and one Awake magazine every month, talks, assemblies, CO visits, and so on.) Who has time to do real research? With just living life, it's hard to find the time or energy to do so, so most, like I did for years, suppress any and all questions. Or, they may think the GB is smarter than they are, so must know better. As my wife keeps asking me: "You don't think you're smarter than the Governing Body, do you?"