Because many other elements also use shadows - some centralnotice banners, dropdown menus, and the postedit popup come to mind - adding a shadow to the main content would help unify the skin and make those look less out of place. And they really should not look out of place when shadows are so effective at non-disruptively highlighting objects.
Adding a box shadow would also serve to bring the content forward in general, emphasising it to the reader/user, as well as help, on poorer monitors, make clearer where the content ends and the navigation begins (on one of mine all the greys are so washed out the entire skin is basically just white with some lines on it, and from what I understand this isn't all that uncommon).
This would be a purely cosmetic change and would probably not apply to old versions of IE, but nor would the other box shadows on various other pieces of the interface.
Basically something like on this: http://en.uncyclopedia.co/w/index.php?title=User:Lyrithya/css&oldid=5653300