Monopoly doesn’t mean “Largest market share”. It’s a real term with a real meaning.
Monopoly:
the exclusive possession or control of the supply of or trade in a commodity or service.
What, exactly, does Valve control? They don’t require exclusivity, they don’t require their DRM, they don’t require the use of their network system. Hell, they don’t even require you to to give them 30% if you sell your own key.
Valve is also not a publicly traded company, while this doesn’t mean you can fully trust them it does mean they aren’t required to seek profit at all costs. This allows then to do things like, support Linux, make their own hardware (twice after their first attempt was a failure), work on Proton, develope games that make them no money, etc.
Itch.io, GOG, EA, Epic, Windows Store, Game Pass, Humble Bundle, personal websites. These are all examples of places you can buy video games on computers.
Timmy Tencent’s propaganda is working on you if you think Valve is any sort of monopoly.