There is collusion between the content distributors and the service providers.
There are only a few service providers in a given area. They package and price similarly. Which is driven by similar demands of the content distributors.
The only "demand" of the content providers is that, in most cases, they want to be in the most homes possible. Yes, that makes the offerings of most providers all pretty much the same. But that's not because they are "colluding" any more than Samsung, HTC and Nokia are colluding when they all offer pretty much the same phones through multiple carriers. They all want the widest market possible. What is so odd about that? It's just good business.
I can buy HBO (or any movie channel) to add to an existing package but there is absolutely ZERO service providers that will sell me nothing but HBO on their pipe. It makes absolutely NO DIFFERENCE to HBO. I can't even buy it and stream it directly from HBO (because their carriers won't allow it). HBO will sell some content that is a few years old via DVD/BluRay.
Who says it is the carriers that prevent it? Remember that right now, streaming access to the premium movie channels is at no additional cost...HBO is simply absorbing the (substantial) cost of maintaining streaming servers. Therefore, they want to insure that you are already paying them for access to their catalog. As soon as HBO thinks they could make as much money selling direct streaming only subscriptions via the internet they will do so.