It's true that I am not a lawyer, so feel free to not take what I say as what the law says. I think that the law certainly should consider Steam to be a monopoly with its level of market power, even if it doesn't currently.

From what I have heard from actual lawyers, monopolies are not currently illegal under US law anyways. They're only illegal when combined with anticompetitive practices. That's my best understanding as a non-lawyer, anyways.