Maybe they don't, but its a huge win for some groups and tends to generate goodwill and news / hype if anyone does anything halfway interesting with it. Valve relies on user trust, there isn't much stopping anyone from using another distribution platform; people have to go out of their way to install steam instead of using the Windows store. Being the 'good guys' is a big part of their business model. I dont want ubisoft managing my DRM.

Also, Valve's highest grossing games are all originally user modifications.