Most of the users are on Steam and the fact that they have an existing library on there is a bit of a network effect. Users will prefer to get more games where their existing ones are. The social networking aspects (having friends on Steam, the ability to stream to each other, join games, and so on) also provide a barrier to new competition via network effects.

Are they a monopoly? I am not sure if it meets the current definition of one in practice, since seemingly no antitrust action is taken on by the government these days. But in my mind Steam is a monopoly for digital game distribution on PC. You could argue that consoles constitute a similar monopoly within each console's platform, I suppose. And you could argue that not all games are sold via digital distribution. But I think we need a new modern anti-trust law that doesn't rely on a very high bar like a "monopoly" but some other less stringent definition.