SteamOS is an open ecosystem. It’s built on Linux. Steam is the default store, but you can install anything.
iOS is a closed ecosystem. I’d prefer it to be open, but Apple has always been up-front about the rules of the game for their platform, so all of the investment everyone has made in iOS has been with full knowledge of this.
PC is different for one critical reason: PC is not Microsoft’s to control; the PC market today is the cumulative result of billions of dollars of investment by PC manufacturers, by Intel, NVIDIA, and AMD, by game developers of all sizes, and by application makers serving hundreds of specialized markets. It would be unfair for Microsoft to use its PC operating system monopoly (see United States v. Microsoft Corp. - Wikipedia) to steal the fruits of the whole PC industry’s labor, by tying a distribution service to Windows and giving it unfair advantages over other distribution services like Steam, Good Old Games, and Epic’s own website and launcher.