That's like saying that Comcast doesn't have a monopoly. Of course there are technically other options, but none of them are real competitors.
The thing with Steam's "monopoly" is that the service they offer makes utmost sense to use. Even if you toss away the whole established userbase thing - For $100, no questions asked (besides being a legitimate company making a game), you get: hosting, distribution, visibility and marketing tools, all kinds of networking and support APIs, etc, and you can freely generate keys you can sell on any storefront, including your own, and get 100% of the profit back, same as you would striking out on your own. You're charged 30% on sales from within the Steam store itself, but those sales are
on top of what you can sell yourself. And anyone from across the world can buy your games at reasonable prices and with a variety of payment options.
So you basically have a choice between going for other platforms that are more than likely to rebuff your project due to curation, picking a free platform with none of the user convenience options or advanced tools at your disposal, striking out on your own for maximum profit but having to do
everything yourself, or paying $100 and having Steam give you the full package of tools and stuff. If all storefronts and platforms on PC were stripped of all their existing established userbases, and I were a developer that had to pick just one storefront to be on with my game, going just by the features and benefits offered versus the costs and risks - I'd almost certainly go with Steam first.
It's something of a "natural monopoly" I suppose, because it's literally the best option for most people to get what they want, developers and players alike. Natural the same way a river is natural - it's the biggest thing around that everyone goes for water to, because it's been here a long time and carved the way for itself, and there's enough space for everyone and plenty of water to go around. Anyone can collect rainwater, dig their own wells or find and claim streams, but there's
already a river nearby, so it makes sense to use it.