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.

Except Steam is neither.

Making comparisons across industries & time periods is not going to work well, especially at a superficial level.