Well, Valve owns Steam, their publishing arm/store/DRM. And like EA now sells their games with their online store/DRM "Origin", Valve does the same with theirs, Steam!
Yeah, they get more by shoehorning people into Steam than they would lose from not selling DRM-free versions.
There may have been a point to stop this from growing into what it's become early on... But it's too far gone to be broken free of save a major scandal. And even then, it'd probably take a few. It's just by far and large a non-invasive tool. They've worked out many kinks.
I'm not saying it's perfect, nor am I saying we should be happy with it. But it's avoided the major problems most other DRM efforts see in "making things more difficult for legit users than pirates". I haven't had any issues with any aspects in years.