The big difference is that iOS/Android are sandboxed, whereas Windows gave every app access to anything. Even if you do manage to install malware on your device, the potential for damage is much more limited.
Apple wants everyday users to have a streamlined, simple, "just works" experience.
It is a common principle in many other facets of life. You can't just buy any prescription medicine for example, even though theoretically a well-prepared patient with access to the newest medical literature could figure out what he needs. But the vast majority most definitely cannot and relies on some kind of access-filter.
I would say the market even prefers the App Store model. Google/Android is working more towards the Apple model and not the other way around.
Devices that can’t (generally) run malware are a value, to some. I have computers, and I also have Pixelbooks and iPads. I use them for different things, and I store different data on them.