Simon - There are those who believe and those who don't. I know that seems obvious but let's break it down a bit more. There are those who don't and never believed and some who are looking for a religion to match their beliefs.
I don't think this is unique to JWism. It seems to be in the DNA of many humans to seek out or imagine a god or gods. Virtually every society in the past invented gods of one sort or other, and that deity (or deities) significantly defined that society. Perhaps humans with the "god" gene had a genetic advantage because they were more easily aligned into large groups or tribes and had a survival advantage over those not so aligned.
To quote Voltaire in around 1770: "If God did not exist, it would be necessary to invent him."