Steam does not have a monopoly by any actual definition of monopoly, though. A) Mobile gaming makes up the most of all video gaming revenue. B) On PC the most revenue is made by games that aren’t even on Steam in the first place (Minecraft, Fortnite, Roblox). Steam’s 2023 revenue has been estimated to be around 8.6bn USD out of 45bn USD of PC gaming revenue. That’s barely a 5th of the market power. By no account this can be actually considered to be a monopoly.
EGS is the Fortnite launcher. Fortnite’s player base is insanely huge. Those people have EGS installed, they just choose not to buy anything else on that platform, except maybe V Bucks.
PS: The installed base of the Microsoft Store and Xbox apps are even bigger because Microsoft is allowed to bundle those with Windows.