Where companies with monopolies are found to gain that title by ousting competitors and brutal buyouts and tactics literally every time, Valve exists. Literally. They just exist. Big difference between a monopoly and the best.

Other companies also exist. In fact there are several launchers and two other digital distributors, and several websites, where one can purchase games. There are some things Steam is shit on. The still feels old interface as a broad example. Competitors could push in, like Epic. Instead, they manage to create the next step up from a gold-tainted dung pile, shit on their own launcher or store stability and performance, and create an experience so bad that Steam is able, through the fuckups of their rivals, maintain a market majority.