With their market share they are by definition of monopoly. Monopoly doesn't mean "only store front", it just means majourity market share to the point they control the market.
People forget there's nothing inherently wrong with monopolies. It's only when they abuse their monopoly position that there's issues.
Steam's customers aren't really the end users, though
Steam's customers are game publishers. Steam provides a service to connect publishers to their audience. Their business model is not "takes money from customers in exchange for goods". It is "takes a cut of each sale that a publisher makes on their platform"
Given that there is no real friction for end users to install Epic Launcher or GoG launcher, is Steam really a monopoly to their customers, the publishers?
If Steam tries to muscle a publisher, they can refuse to publish on Steam and still have options. When popular games aren't on Steam, it does seem like people have absolutely no problem installing another launcher/storefront to access it
Look at the massive success of Fortnite, which is only on the Epic Launcher
Steam is also a monopoly but it doesn't abuse it's monopoly. E.g. impose terms like games can't release on other stores for a lower price &c.
Nothing stopping a competitor from coming in with a superior game store.