I can think of one simple reason (not necessarily a real good one, but simple): confusion. People see an HDMI input and assume it will provide an audio input, especially as the standard evolves and it starts appearing in more places. If they have an HDMI port that is actually a DVI port with an HDMI adapter built in, you'll have people who don't realize the subtle difference (there's a reason for the "F" in "RTFM") and get all bent out of shape.