The reasoning behind these kind of differences is what makes OS X different from other OSes. The MS Word behavior you describe was born in (or at least comes from) Windows. The Pages “page” allows you to resize the window and make the content fit the size, therefore occupying the entire document window. Since you can option+cmd+P to show the pages thumbnails, my guess is that the way Cocoa control works makes more sense if the page is left-aligned. Since under OSX you can only resize a window from the lower right corner, this also means they don’t have to worry about keeping your page visible if you perform a fancy resize from a different place (because you can’t). Lower right means you’ll just be making the visible area smaller, but never really “covering” part of the horizontal part of the page, and if you do, Pages will make it smaller to a certain degree until it won’t let you make the window smaller (horizontally speaking).
Could all this be made if the page was in the middle like MS Word?
Possibly, but it would have looked much worse and the space on the left of the page would have been wasted space. You would also have a “moving” page when you resize if you had previously zoomed in in order to accommodate the contents in a proper way.
If you keep you document window the size you want to see it, I don’t see why you need the page in the center, since the MS Word border you talk to, is a wasted screen space. If you want to see the text in a more “relative to a real page” thing, you can always use “Page Thumbnails” (and make it bigger) or turn on “Show Layout” shift+cmd+L, but that’s as best as you can get.
I really never thought about this difference until I read your question, but after using Pages for a couple of years, I can say I like the Pages’ approach better.
It’s a matter of window management and operating system differences I guess.