Two reasons:
Your mp3s and avi sources are "compressed." In other words, by limiting the frequencies a file has a play, the other frequencies can get louder.
A DVD outputs very high quality audio, usually 1-48KHz, maybe higher. A 128kbps mp3 might play from 20-16KHz and a brick wall filter outside of that range.
Another reason is, DVDs aren't wave sources, whereas computer sound cards boost wave sources by +12db.
Essentially this is why there are home theater speakers, with multiple drivers, and large, firm speaker cones and tweeters that can play a wide range of frequencies accurately, and with a lot of power and gusto, with low distortion, to tackle the issue of accurate reproduction of a higher quality, more accurate source.
While 20-20KHz may be enough at low listening levels, part of the home theater experience is "feeling" the sound, which is why the range extends to a minimum of 48KHz (as well as other reasons). Our perception of the loudness of sounds increases as the volume increases (what I mean here is, our hearing range expands as the volume of those frequencies expand). A 60db 20hz signal is perceived as loud as a 20db 3,000hz signal in our heads. But as the volume goes up, our ear's ability to "nullify" frequencies decrease. When you get up to 100+db, our perception of a 20hz signal is the same loudness as a 3,000hz signal. So more resolving speakers, and a higher quality source is needed for HT, which operates at very high volume levels.