It's because they're agnostic, and as we all know, agnostics are the worst people in the history of the Earth.
Ask Buddhists what's up with this business about the reincarnation of the Dalai Lama, for example, and you will receive long replies which, to be sure, in no way answer your question or are good for anything else either, but point out how you supposedly don't understand something -- could be reincarnation, could be the concept of reincarnation, could, be Tibet, could be all sorts of things. This is the essence of agnosticism: insisting that everyone else doesn't understand, and being useless.
And nasty. For example, read this. I'm sorry. I promise I won't link that bad man again. (He's agnostic.)
PS, 16. September 2012: Actually, he's an atheist, and agnostic on the question of Jesus' historicity, although leaning recently more to the historicist side. And he's actually not so bad. He was severely provoked when he wrote that. You should check out some other things he's written.
PPS, 22. October, 2014: Actually, I've heard that, although he doesn't believe that God exists, he doesn't consider himself an atheist, but rather a skeptic. Yeah, that doesn't make sense to me either, but apparently he's not the only one. Just as I consider people who call themselves spiritual but not religious to be religious and confused about what the term means and/or in denial, so I consider people who don't believe that God exists but to be atheists, and to be tiresome when they deny that they are. I still think think his writing is very much worth reading.