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 <i>ne verb pas</i> negation in the context of strong emphasis, the grammaticized <i>pas</i> having completely replaced the original negator <i>ne</i> in day-to-day speech. Only tweets, text messages and smilies will prevent English from being as static as written "Chinese".