First off, since the web mostly works with HTML pages that can only be updated via request/response actions (I'm ignoring flash), you can't really have a model update the view's output directly. Also, there are so many possible views that updating them all would be very inefficient.

Unless you get serious about AJAX (many people bet on this). In theory, you can implement the entire MVC in JavaScript (client-side) and have your server acting just as a Web Services provider. In fact, XForms is just that, the only problem is that it still isn't supported by the web browsers..