That’s like saying Walmart has a monopoly on what they put on their shelves.
No, it's like saying that Walmart controls the furniture in your house. It's your house - you own it, you pick what furniture goes in it.
Similarly, the iPhone is yours. It should be up to you what apps you want in it and where you get them from. You want to exclusively get furniture from Walmart for your house? You go ahead - no reason anyone else should be forced to do that. You want to exclusively get apps from Apple? Go ahead - do the same thing on your Mac.
Have you installed anything on your Mac not from the macOS App Store? The absolute horror! Why would you ever do that!? Oh wait, yeah, there's tons of great macOS software not on the macOS App Store. Similarly, there'd be tons of great iOS software beyond what you can currently get on the iOS app store. But there's no easy way to get it, because Apple set up barriers that benefit absolute nobody but them, and so that software simply isn't developed. People say "wouldn't it be great if the iPhone could do that?", remember that one of Apple's hundreds of arbitrary rules that benefit nobody disallows it, so that app doesn't get made. Or maybe it's an Android exclusive.