As mentioned briefly above, the reason most lavs don't have windows is that in all newer airplanes they are modular in design, so the airline can place them wherever they want in the aircraft. This also allows for easier maintenance as the entire unit can be removed and replaced if there is a problem with it. All it takes is flipping a couple of latches (and unhooking the water/waste lines) and you're free to pull the lav out. In some locations (usually first class) the position won't change, so a few airlines do decide to spend the extra $ and install windows.
A couple years ago I even witnessed an incident on one of our DC-10's, where a new FE was operating the main entry door on the left, unaware that the lav just aft of the door had been worked on the previous night and had been left a couple inches out of place without being locked back down to the floor. The door came down on the lav and in an instant ripped one side right off of it. We had to take a delay until the mess could be cleaned up.
One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.