I've been thinking about that... It's true that Steam doesn't have a monopoly in the conventional sense, you can buy "Steam games" wherever and Steam doesn't see a cent (not 100% sure) but... Steam does have a DRM monopoly, you can only play many of the popular games using Steam as a platform, there is no choice at all.If you don't download their client, and make a "Steam account" you're screwed. Kinda like Sony has a "monopoly" on Sony games and likewise Microsoft with "Xbox" games.
Also it's different with consoles, you buy a *Sony* console and expect it only playing *Sony* games (which is true, they all say Sony regardless of dev or publisher)
You don't buy a"Steam PC", you just buy a PC (which does come with some limited choices admittedly )
So while that's debatable these are really monopolies, I think that's what people mean when they say monopoly, you *have* to be in their ecco system.
And I also think that's why people like to shill for the Epic store, it's "competition" the worst possible competition imaginable, but still competition - with the same policies and goals as Steam ironically.