I don't know - but they are.
People don't seem to worry much about being wrong about the truth of many other things - but they really, really don't want to believe Christianity if there is any chance they may be mistaken.
Sometimes the reason given is that Christianity is too good to be true, other times that it is not good enough. Either way they don't want to believe in Christianity unless they can be 100 percent certain with zero possibility of error.
Strange but true. Of course, on such a basis, a Christian is the one thing in the whole world that they never can be.
Note - This post comes from reflecting on 'Pascal's Wager' - which I think is, at root - and considered in context, concerned with exactly this matter.
http://www.gutenberg.org/files/18269/18269-h/18269-h.htm#p_233
