Here's something that I learned working for a cable company: HBO doesn't really know who their customers are! Or to put it another way: HBO's real customers are the cable companies. As far as I can tell, HBO never deals with subscriber data. That's why you can't sign up with them directly.

I know this because I know how HBO GO works at my company. The subscriber visits HBO's site, but then they have to tell HBO which cable company they have. Then HBO redirects them to that company's website! After they verify with the cable company that they are an HBO subscriber, the cable company redirects the user back to HBO with a "subscriber" token.