Because Modern English is a bastard language which, in 1066, began to mashup the old Anglo-Saxon and French languages (the populace spoke Anglo-Saxon, the nobility spoke French due to William the Conqueror). With the addition of Greek and Latin words, along with the expansion of the British Empire, tons of words from tons of languages came in ("skunk" comes from Algonquin in North America, "tea" comes from Chinese). The words retained their original spellings (more or less).
A movement started up in the mid-Twentieth Century to standardize spellings, but it would cause as many problems as it solves. Pluralization is largely by adding "s" to the end of a word, but sometimes it's pronounced "z" due to a letter nearby (keeping it the "s" sound would be difficult to impossible - try saying "birds" with an "s" sound instead of a "z" sound). So fixing the spelling issue would create a new level of nightmare with grammar.