Unlike the offline world, many disabilities are NOT obvious even when interacting with someone in a live chat or via email. Georgia Tech asked this question in their regular surveys of Internet users, and the results were pretty high. The numbers will only increase in the future, as Internet access becomes more widespread AND as the average age of the web user increases. Speech synthesis, voice recognition, braille terminals, and a wide variety of specialized input devices. Unfortunately, though, this isn't the case, for one simple reason.
Technology can only provide whatever information it's been given.
That information MUST be supplied, and it's up to the author to dilligently provide enough information to make the web page usage.