Their key advantage is specifically dominant market position.
Whenever the topic of monopolies come up, people just default to black and white thinking. The legal reality is a whole lot of gray. Simply being a dominant player that offers more advantages for people to use the service isn't going to reach the standard for any anti-trust regulation.
Think of it this way: In the late 1990s, had Internet Explorer
organically become the leading browser and people chose it without coercion over its competitors (and there were several), there would have been no case against Microsoft. However, because Microsoft was behind the scenes manipulating people
into using it surreptitiously via their marketshare and OEM contract leverage, they suddenly found themselves on the wrong side of "monopolistic practices" anti-trust litigation. It wasn't that they had become dominant, it was
how they had become dominant.
To my knowledge, Steam/Valve hasn't leveraged their market position to
force others to use it over any other marketplace, or to
force others out of the market entirely. But that's a legal topic for someone more well versed in law. Maybe there's a case to be had, but I think it would be a tough one to prove.