zilch0md has it right. Not only are most music customer's not looking for audiophile DAPs, they don't even know/care that it will improve the quality of their listening. They assume the headphone handles that end of it.
In addition to that, having worked in retail for a good chunk of time, buyers for these stores surely don't even have the brands WE know and love on their radar.
This gets back to another, crucial point: audiophile gear has been around for a long time - much longer than the envelope of apple products - and it's always been a niche market. Most people listen to music for different reasons, and with different focus, than folks like us. Even when I show off my equipment to the friends of mine who otherwise listen to their white earbuds and are amazed at what they hear, they're not of the opinion that they can't go back to their low-fi equipment, and they're certainly not going to shell out the bucks like some of us do (possibly far too often.)
It's not a supply-side issue as much as a demand issue. If there were half a billion people clamoring to spend $200-$1200 for a quality DAP, surely the bugs would be worked out a bit faster than they have been so far. The cell phone, whatever we might think of it, is overwhelmingly the audio source of choice.
Well written post - the bolded text nailed it. I drop in here every now and again. Your post nailed the frustrating but true aspect of it. I use an iPhone 64 gb and a Centrance HIFI-M8 when I need to drive headphones portably, or want to run the phone into an amp. I use the iPhone alone with my ciems. I have yet to use a a DAP that is "better to me" than an iDevice. The AK's come close, but the value proposition isn't there for me.