"Gamers" tend to be an apathetic and lazy bunch. It's either too much effort for them to put a disc in tray or too much effort to use a start menu or a desktop shortcut.
As long as they have no problem in their little bubble they couldn't care less if other potential customers have issues with connections, isp's, bandwidth caps or simply see no reason to support drm and the erosion of customer rights.
If Valve has a monopoly on PC gaming with their obnoxious and *for me* totally unnecessary drm client then I'll just stick with the games I already have.
All steam has done is mollify an easily pacified customer base that frequently forget how to be customers with rights.
While I do agree with this - I've been increasingly seeing people actually too lazy to get up out of their PC chair to fetch a disc to install a game - I think laziness is only part of the story.
And yes, I'm aware that sites like Steam are a boon to those living in countries where buying games legally is a genuine chore - by which I don't mean walking 10 minutes to a store, but rather in countries where they have to import games at great cost.
But we shouldn't forget the fear factor. A lot of hardcore PC fanatics have been painting Valve and Steam as some kind of saviour of the PC gaming industry out of some kind of fear that if Valve did not exist, PC gaming would cease to be. They are worried about a lot of the majors abandoning the PC as a platform, and place all their hopes in Valve out of some misguided notion that if Valve can be sustained, then PC gaming can be sustained.