Why do people expect coherence in the bible?

Lots of questions on this site are asking how some text from the old testament fits to a contradictory one from the new testament or a passage of one book to the other. Why do people expect to find this kind of coherence in the bible? Clearly the different text passages are written several hundred years apart, by different people with different interpretations of the exisiting scriptures in mind and with a varying set of personal beliefs and values.