If HBO were to go internet-only, it'd lose the major cable operators heavily promoting them for free. They only market premium channels so hard because it helps sell expensive cable TV service. HBO would lose subscribers immediately as operators no longer actively replace subscribers for them as others churn out.
To make up for the lost stream of customer acquisition, they'd have to:
a) Cut prices by more than 50% to become competitive with Netflix/Hulu. This will cost them billions a year.
b) Increase their catalog size significantly, which would cost billions a year.
To give you an idea of the costs of licensing, HBO's last major deal was with Universal studios and cost them over $2 billion for access to their future movie catalog.
It's hard to come up with a scenario where cutting their exclusive tie with cable makes sense financially right now. It's not "bullheaded" to say "no, we shouldn't throw away $10+bn/year to give Bob his internet subscription while we're in the midst of a death battle for the future of digital entertainment distribution".
Netflix can do it because (a) they have a big catalog and low prices to entice subscribers with, (b) they got much of their catalog on the cheap before studios fully understood its value; this perk is going away as licenses renew, (c) it has no choice. Whether Netflix can grow its original content business within the constraints of its low subscription price faster than its licensing costs get out of hand is still up in the air.