"Chinese" is a special case, in that there are actually a large number of spoken languages (of which Cantonese and Mandarin are merely the best-known outside of the region) that share a common system of writing that is largely independent of the sounds of spoken language(1). Imagine, if you will, that a person could write something in French and someone else could read it as if it were Italian, Spanish or Romanian without a hiccup. I18n would be a much easier task if European languages were so encoded, no doubt, but literacy itself would become a much more remarkable accomplishment.
(1)A large number of Chinese characters are composed of a syllable radical which originally acted as a guide to pronunciation (say, "ma") along with a context indicator which would tell the reader which of the various "ma" syllables was being used. In the original written language, it probably made perfect sense, but language drift over time have left the system essentially arbitrary in most modern Chinese languages.
[EDIT -- tried to use an asterisk there, forgetting that paired asterisks result in italics.]
Good question -- and the answer is that systematic writing combined with widespread literacy tends to fossilize language, at least on the page. There is, for instance, a world of difference between written and spoken English, the written version being not too very different from what one might have expected to read in an eighteenth-century document. The vocabulary has changed somewhat, the modifiers less (ahem) magnificent, but the grammar (and the flow and phrasing) is essentially the same. In fact, it is really only semantic drift that prevents us from reading Shakespeare as easily as, say, Tom Wolfe. Even the spelling in today's written English reflects the pronunciation prevalent in the London area in the fifteenth century. None of us, though (or at least none of us who regularly interact with other English-speaking humans) would speak spontaneously in any way like the way we write. The situation is similar in other languages; in spoken French, for instance, one only hears the ne verb pas negation in the context of strong emphasis, the grammaticized pas having completely replaced the original negator ne in day-to-day speech. Only tweets, text messages and smilies will prevent English from being as static as written "Chinese".
