Because people still buy them anyway, and have no way to objectively determine which one is better ?
By the way, I doubt that "Any of the ones supported on Head-Fi would be better, hands down" than the Xonar. Here are some test results for the Xonar D1, which is the same card, but with a PCI interface. Check also the Sound Science forum where a number of other tests (ABX and difference extraction) feature this card among others. A PCM270x based DAC will measure worse even with a "perfect" implementation that achieves the datasheet performance of the chip. Also, the first suggested ODAC is more of a sidegrade with similar specs. Now of course it is possible for an internal sound card to have interference problems in some computers, but if that is not the case, then improving on it with a DIY DAC is not that easy. And even if you do succeeed, the difference would be difficult to hear at best under controlled conditions (try some of my ABX tests if you do not believe). Nevertheless, DIY can be worth it simply for the fun, so I am not suggesting the OP should not bother trying it at all. Also, when you spend so much time building something, and are proud of the end result, it will quite likely sound better subjectively.
I do not quite understand what you said, I'm sorry :(
you are saying that no advantage would be to replace the asus xonar dx?