I think it's probably done partly for aesthetic purposes. It's cleaner and in some cases more usable. Having a login box on every page can clutter the screen. And having to find a sign-in button hidden in the corner of the layout every time you want to log in is less convenient than simply being able to bookmark the sign-in page.
This isn't a site like StackOverflow or Facebook where you have a content-based site that doesn't require signing in to use. You have to sign in in order to access the service, so the public and private sections of the site are completely separate. It's not like the user is going to sign in and then stay on the current page to post a comment.