True, and also because a lot of people have invested time & money into Steam. I like to keep my games in the same spot as much as companies allow me and Steam will not be dethroned in that regard.
And I really don't want my gaming habits to be broadcasted to my Facebook friend list. Why? Gaming still has a social stigma attached to it. My close friends & relatives know I game, but I don't need to have the majority of my Facebook acquaintances to know.
- - - Updated - - -
The Origin niche works because certain games are Origin-specific (like Dragon Age 2/Inquisition). If this Facebook platform won't have an exclusive contract with a publisher, then I don't see how it will even become a niche, except for the Farmville crowd.