There are many, many AR's (aspect ratios). Some I've come across include 1.77, 1.78, 1.85, 2.00, 2.35, 2.37, 2.39, 2.40, and I'm sure there are many more.
Sometimes they film the movie with a larger screen, so to speak, and the producers/director will decide to crop some image.
Sometimes a certain film will have an AR that itself is cropped for the DVD/BD.
Sometimes a movie has multiple AR's, the Dark Knight being a prime example.
If you zoomed all the AR's for full screen, you obviously are missing some of the picture. In certain cases, maybe a LOT of the picture. Can you imagine watching Lord of the Rings, taking out a whole lot of the image? Do you blame Peter Jackson for going with the more cinematic, wider AR of 2.40? Nah, I'd rather he design if for the theater, and not my neighbor's 42" tv.
Some persons use masking systems for the PJ so that it always looks like a full screen.
There is even some new TV, maybe it was a Runco (??), that has a 2.35 screen or something like that. Costs more than a car, and of course you'll still get black bars on the sides when watching more squarish AR's.
Don't blame the movie theaters for not designing the screens based on our old CRT tv's.