...A monopoly prevents competition. Valve hasn't. What exactly do you think that they have monopolized?
Try finding a copy of Napoleon: Total War, Civilization V or Half Life 2 that doesn't require you to use (or create) a Steam account. That should refresh the memory of what a monopoly is.
LOL. That's not a monopoly. You still do not get it, do you? Valve does not own the rights to the game (except for HL2), the publisher does. If the publisher chooses to use Steam as their only outlet for gameplay, that's their choice.
The ONLY way that would ever be considered a monopoly by any court, regardless of country, is if those software titles were required to run on something like 50% or more of the world's computers. This is why Microsoft got into trouble with Internet Explorer. Are you required to run those games? Uh... no. There's no argument you can possibly come up with where anyone is going to reasonably agree with you that you have to be able to run those games to function. That is not a Valve problem (again, other than HL2), so take those problems up with the publishers. You might as well say that game stores that have exclusive DLCs have a monopoly on those DLCs. "Exclusive deals" are not the same as a monopoly.
And on the topic of Civilization V, you don't have to buy it through Steam. Yes, you have to run Steam to play the game, but once more, you don't have to buy it through Steam. Monopolies are ONLY monopolies (by definition!) if you can only BUY from one entity.
Stop confusing what you think should be considered a monopoly and what is, by definition, a monopoly.