Imagine thinking that Valve has a monopoly.

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.

Just like I am happy with Apple and Google taking a cut and running their app stores. If these big companies could make their own store, they would. Apple would lose a cut, but that does not affect me as a consumer. What does affect me is a gate keeper keeping terrible practices in check. Making it nearly impossible to cancel a subscription instead of having a handy menu to just turn it off. Having places to put credit cards that are not secure. Collecting personal data nonstop. Etc etc.