For me, Steam Workshop is the killer feature for a bunch of games. It's hard to beat zero-effort auto-synced modding and zero-effort mod accessibility for tech-illiterate friends.
While Steam itself certainly has DRM infrastructure, it's not required and easily bypassed. Valve allows other 3rd-party DRM solutions, but I'd argue that the built-in DRM is one of the less important aspects of steam.
If a game gets content updates every other month, re-downloading a pirated game, and possibly losing all progress, meh.
If a game is going to go on sale for $20 (or $10) after awhile, why bother pirating it, just wait.
And Steam is absurdly convenient. Built in voice chat, collectables, and forums, means playing games on Steam is better than playing pirated games.