It has to do with the way Valve has approached it. From day one it wasn't, "How do we stop people from pirating?" It was, "How do we make a more convenient service than the pirates?" And that's where they won. In the early days, people pushed back. But then the brand of Valve as people won out over the brand of Valve as the makers of Half-Life. If you make a good product that has convenience over the alternative (piracy) and charge a reasonable amount, people will pay when they used to pirate. The DRM is incidental to the experience. Valve realized it first, and now you're seeing it replicated in things like Netflix and Hulu.