I agree with you but I have this nagging feeling that it's more complicated than that. I wouldn't be surprised if Valve was nearing dangerous antitrust territory with the dominance of the steam platform.
It's much less dominant than the Apple store (which really is a monopoly on the iOS platform). If there is an anti-monopoly case it will be there first.
Where is antitrust? There's Origin and anyone (i.e. any publisher or indie gamer) is free to make their own store online anytime. There's monopoly where the cost of entry is high, but in that case, the cost of making an online store to sell your own games is close to nothing. There's also Desura, Humble Bundle (they don't only sell Steam keys), and lets' not forget the plethora of online and retail distributors which sell physical games still.
Anyone is free to make their own operating system, browser, and bundle them too, yet Microsoft got pummeled by both the DoJ and EU. AFAIK (IANAL), the market dominance of a product has a strong effect on whether something can be brought under the jurisdiction of antitrust law. You're right though, as the cases I've seen mostly revolved around anti-competitive tactics, like Microsoft above and Intel for their OEM deals (both of which I think relied on their market dominance).
Unfortunately that's not how antitrust laws work. How a company establishes a large market share is not a necessary concern in historical court cases. Merely that a company has a large market share seems to be the primary consideration.
