Because cachet is a vital part of Apple's marketing strategy. If they could capture some of that kind of attention for doing something respectable, they would benefit.

Plus they have additional constraints on them because they're using Bluetooth.

That is significant. However, Bluetooth is improving. Other forms of technology are improving as well. We're getting to the point where very linear drivers combined with digital processing are starting to make a difference. Soon, we'll be able to make such devices cheaply, with sensors that are fast enough to make regular consumer devices perform like today's high end headphones.

Apple has traditionally cared about aesthetics enabling technology. Their screens are good, and while not high end with sound, they have sold decent sound hardware in their laptops. They were early with Toslink. Steve Jobs cared about decent sound as well.