Just having a high market share isn’t the issue. It’s abusing that dominant market position that is.
Valve has been smart enough not to do that. Google, Amazon, Microsoft and the like haven’t. In fact, Valve’s competitors have been more anti-competitive than Valve.
ASML, who make EUV machines and other semiconductor tooling, is also in a dominant market position (way more dominant actually). Do you ever see calls to break them up? No. Because they haven’t been abusing their power. They know that if they put a toe out of line, they’ll be in trouble with regulators.
Google and the like have been able to act with impunity because the US protects them, to the detriment of their smaller companies and their citizens.