Xbox Live has 40 million members. They aren't all Gold members, but even if they were all Gold members and even if they paid full price for their subscription, that is $60 x 40 million people. $240 million dollars in yearly revenue. Even if every penny of that is fed back to publishers that's not a lot of money considering that game budgets have spiraled to $80-$100 million. I'm not certain why people think publishers would be satisfied with this meager revenue stream as compensation for every single copy of their game being shared with up to ten people.
If every subscriber shares just 2 games with 1 other person, that is (in a publisher's eyes) 80 million units lost in sales. At $60 per game, that amounts to $480 million in lost revenue. More than twice what they would receive if they took 100% of XBL Gold revenue. And that's a pretty generous estimate considering that most people will share way more than that.