why does overwhelming market share matter?' and the answer is 'because that's a monopoly.'

That's a tautology. It would be equally relevant to say "why do monopolies matter? The answer is because that's an overwhelming market share."

Neither the word "monopoly" nor the phrase "overwhelming market share" inherently share the concerns of that situation. You've even been arguing that the using the word monopoly doesn't actually matter: "do nothing about it, keep doing what you've been doing is fine".

And to be clear: are you saying to disagree with the fact that Steam does not satisfy the dictionary definition of monopoly? Or with that fact that people are more likely to be familiar with the dictionary definition rather than the legal definition?