You can go back in history and apply that logic to any company and claim they weren't monopolies as a result. Standard oil and AT&T had plenty of competitors, none were able to grow beyond extremely small scale.
The key question is whether an antitrust target is engaging in uncompetitive practices. I haven't noticed any claims of Steam/Valve using uncompetitive practices (but I could have simply missed them.)
Whether something is a monopoly and whether they've abused it in such a way that they should be struck with an antitrust suit are two different things. I don't think many would argue that Steam abused its monopoly.