Written characters and sounds don't usually correspond because pronunciation drifts very quickly in time (even two consecutive generations might have noticeably different pronunciation), and in different directions in different locations, while rules of "proper writing" change rarely, usually as a result of people agreeing to change them in order to catch up to an apparent accumulated drift of pronunciation.

As I understand (in a simplified way), every once in a while (once in a few centuries) there's a major language reform to make written form reflect current pronunciation as much as possible, but already in 100—200 years people would start asking "why do we write not like we pronounce", until some political movement decides reforming orthography would promote their agenda. Then the cycle continues.