Why Diagonal?

Why do they give the hypotenuse and sometimes the slope (aspect ratio; ie. 16:9 or 4:3) of a tv or screen instead of giving the dimensions? Most of us don't deal with the ratios of TV screens all the time, and can't instantly tell you what the height and width of the screen will be from just the diagonal measurements. Sure, it is a simple calculation to figure the other sides of a right triangle based on the hypotenuse (diagonal measurement) when given the slope, but WHY? It is much easier to say 6.1" x 3.4" when talking about a so-called '7" screen' than to give the 16:9 aspect ratio ONLY. Maybe its because 7 inches sounds BIGGER than 6.1 x 3.4 inches?