Well - the rare ones do happen - Where they do come to believe in God - without any sort of illness. Just had to stick this point in there ... wasn't sure if you meant the post literally.
Yes, I perhaps there are some. I know a young woman who grew up going to a super-groovy Unitarian church, very, very liberal, very, very non-god oriented. She told me that after her grandmother died she became religious. First, because her grandmother visited her in a dream so she started thinking there might be an after-life. (IMO, such dream visits mean nothing of the kind, but, anyway..._ Second, because she felt that that there was no ritual or anything that helped with accepting and mourning her grandmother's death. She now goes to a United Methodist church. It is a very, very liberal church to which she belongs and she isn't at all into preaching her faith ay anyone...so she didn't flip out and go all holy roller, tongue speaking.
It seems that as humans, we have a near-infinite capacity to delude ourselves. Is open-mindedness natural, or is it more of a learned skill? Regardless, the indoctrination that most people who are brought up with religion experience can be quite an effective wall against reality. Belief in heaven and hell combined with a mandate to believe is dangerous. It's hard to contemplate an alternate reality when the mere contemplation can be seen as grounds for eternal punishment.
As far as leaving us be, that would be in direct violation of one of the key tenants of their religion. I believe that's part of what makes religion dangerous. They'll never be satisfied until everyone believes.
Improbability, that is the key word here.
Because we (atleast until now), cannot disprove the existence of god, theists will always have something to argue about. And there isn't much we can do about it, except hope that education, better education, will in the future help rid the world of such thinking.
What is more annoying is the negative reaction atheism get. People, by and large live and let live when it comes to religion. I have never seen anyone tell someone following another religion that they are wrong. But people don't grant the same respect to atheists. Even if they don't outright tell us that we are wrong & will goto hell, there is a visible shock on faces when told that they are in presence of an atheist. Its like believing in a religion, any religion, is the default position and being an atheist is something perverse.