Originally Posted by BrianO

Because the movies are put on the HD-DVDs and BDs in their original theatrical aspect ratios. The MPAA is dead set against the cropping of movies shown at HD resolutions.

Almost all movies today are filmed with aspect ratios of 1.85:1 or 2.35:1 (2.39:1 and 2.40:1 are exhibition variants only). Both of these aspect ratios have been in common use since the 1950's. (It is not something new and it is unlikely to change as far as film is concerned). Both are wider than 16:9 (=1.78:1). 2.35:1 movies will always show bars and only fill about 3/4 of the height of the 16:9 screen. A 1.85:1 movie only fills the screen of most 16:9 TVs because of the overscan built ito the TV. Without the overscan, 1.85:1 movies will also display black bars, albeit very narrow ones.

On older movies you will find many other aspect ratios used, some as high as 2.76:1 (MGM Camera 65/ Ultra Panavision 70).

It is a common misconception that the 16:9 aspect ratio was chosen for TV to match that of movies. It wasn't. It was chosen for technical reasons related to TV broadcasting and data compression requirements.