Why Can I Only Burn 80 Minutes of Music to a CD if My MP3s Take Up Less Than 700MB of Space?

When burning a CD, you can either burn it as a data disc or an audio CD. A data CD can hold up to 700 MB, while an audio CD can hold 80 minutes of sound. If you have 200 MB of MP3 files that add up to three hours of music, you can still only burn 80 minutes to the disc. Why is that?