Because a monopoly is a company that operates in their market unopposed. In this instance, it’s not Steam’s fault the opposition kinda sucks (or doesn’t quite aim to be a direct competitor in the case of GoG) but the argument is still there that Steam is sitting as the only distributor for PC.

Personally, this is why I keep wanting to root for GoG, Epic and such. Monopolies are dangerous to consumers and the markets they operate in. Right now, Steam is being surprisingly effective at remaining a “good guy” but there’s a lot of concern even among Steam fans of what the landscape will look like in a post-Gaben world. Setting the PC gaming market up to have Steam as the only option when that inevitably comes to pass (touch wood that that’s no time soon, of course) could spell a certain level of disaster in a world where the anti-monopoly law-makers have shown to not really care about upholding that standard.

Edit: missed words. Never type when struggling to keep your eyes open kids!