You don’t need to have full control of the market to be considered a monopoly, you just need a big enough share that you can make it sway in the direction that you want, which Steam has. Example: Microsoft is considered a monopoly even though there’s Apple and Linux that get market shares.
I always find it funny how defensive people get when I bring this up about Steam on Lemmy of all places, suddenly people are perfectly ok with the centralization of power in the hands of a single person.