Is there a technical reason why I cannot find a mini display port to hdmi (female) cable that's more than 6 inches?
May I suggest that you not look for a cable with a "female" HDMI end on it (those would typically be called "adapters" and would be very short.  Instead, look for a full-length cable with mini-DisplayPort to HDMI.  Such as:

http://www.monoprice.com/Category?c_id=102&amp;cp_id=10246&amp;cs_id=1024603