Because you still can never prove them 100% wrong. There is always that 99.9% chance that you are wrong and that little .01% makes a world of difference. You know, instead of ranting against them, have you ever thought that perhaps the Christians are wrong, but the book is actually right? All you'd have to do is read it and make sense of it yourself.
Deciding to rant against a large group of people because you have a problem with, most likely one or two doesn't make sense and will only tire you out.