Steam's DRM has nothing to do with why individuals buy games on Steam. Steam offsets the net negative of DRM by bundling in a number of very attractive value-adds, like the ability to reinstall your games on any system without media, keeping all your games up to date for you, syncs settings and such (for Steamworks games) and provides an attractive platform for communicating between friends/arranging games and such. Not to mention that they have great sales. (For me, the first one is the killer feature. I hate playing the install + patch game.)
Now, they could easily provide all of those things without adding any DRM at all, but they don't, presumably because neither they nor their customers (which, make no mistake, are primarily the companies who distribute games through their platform) want it enough to cause it to come to pass.
I mean, I love Steam, it's incredibly handy and has singlehandedly transformed the way I consume games. But let's not pretend it's something it's not.