Because A, it would look like crap. B, you can't change from day to week to month at all. And C, you can't view the details of items in your calendar. I think what this guy is looking for is a transparent calendar of some kind, not taking up the whole screen.
One program I can think of that does this is Rainlendar, which offers a month view and also shows appointments I believe. It only takes up a few inches of your screen, and can be transparent as well.