It would be nice if operating systems would be exactly that, but for some reason "commercial" OS vendors are not able or willing to provide a safe "native" sandbox completely uncoupled from curated app stores.
Apple is going down this route with code signing and granular permissions with Macintosh applications that don’t have to be part of the Mac App Store and there is still whining from geeks that this is leading to a walled garden, even though there are approved ways to work around it.
It's the same with Electron apps by the way, those wouldn't exist if operating systems would come with cross-platform UI frameworks that are actually better than the unholy mix of HTML+CSS+JS. But Microsoft, Apple etc... had decades to get their act together without delivering.
One of the problems with Electron is that it’s the same interface everywhere. I don’t want the same interface on a Mac that I have on Windows.