Why? Because the file size difference between hq sound and crappy sound is tiny relative to video. And I think most people who have worked in audio/video would agree that sound is more important. A video with poor sound is unwatchable.Why? The reason both streams are overcompressed is the same: cram more channels into the available bandwidth. Many people aren't as concerned as we are about PQ, and even fewer IMHO are concerned with AQ.
I disagree that fewer people are concerned with audio quality and believe I could prove it in real life with a simple demonstration. Just because people don't talk about it doesn't mean they don't value good sound. It's the type of thing you don't notice, until you do.