It's because audio on cd's is defined differently than it is on computers. On a computer, you have some kind of file setup like an mp3 that the music is stored in. On a CD, you have raw audio. If you get down to the nitty-gritty of a CD player, it works much like a microphone (well, in a sense). The laser eye (which is actual a laser and photosenitive plate coupled together behind a lens) reads the CD by firing a laser at the CD. If the point on the CD it shoots at is reflective, the laser comes back down at the same angle at which it was fired at the CD. The lens then refracts this back to the photosensitive plate. If the place it's shooting at isn't reflective then the laser obviously won't be reflected onto the plate. The plate basically works with a transistor to make disturbances in the flow of electricity that goes to the headphones (well, the amp circuit). So, the audio recording of a CD is really the raw sound in analog form, not a digitally quantified form which allows you to have that 80 minutes of song on a CD.