I have an Asus STX II dedicated sound card and it is a very substantial improvement over built in sound cards, to the point i'd never consider having a PC without one. Most people have just never tried anything better and thus live under the illusion that the built in sound cards are "good enough", rather than the lowest common denominator.

Lack of marketing, maybe? A myth that you need to fork over thousands for a complicated audiophile setup to get very incremental improvements over the baseline? Probably a combination of these is to blame.