That's because on DVD Video, the "full image" is actually inherently 4:3, either 720x576 pixels (PAL) or 720x480 (NTSC). 16:9 modes are specified as well, but tend to go unused: Widescreen material is saved in 4:3 as well, exploiting the greater vertical resolution this affords, and un-stretched during playback - this technique is referred to as anamorphic.
The Blu-Ray spec, OTOH, doesn't specify a 4:3 HD mode. It supports the same 4:3 SD modes as DVD does, but if you want to store a HD image, you have to use a 16:9 aspect ratio. Thus they had no choice but to encode the black bars into the image.