The Steam DRM isn't even good at this point by the way, I don't know why people think they would choose it for that alone. It's a known quantity in the pirate community, they have their own packed installers for Steam games and often have private server executables and etc.
If they were going to choose Steam for something, it wouldn't be for the DRM. Like Jerykk said, Steam is just very popular, accessible and has features desirable to the majority of their player base.
Playing: Dark Souls 2: Scholars of the First Sin Watching: Mr. Robot