> Why not just run all desktop apps in sandboxed virtual machines then?
Ideally, we would like sandboxing on the desktop to be at least as good as sandboxing on the web (preferably better). People don't run sandboxed desktop apps right now because the ecosystem currently makes it inconvenient. Wayland and Flatpack are both good steps in the right direction. Apple's making some progress as well there, but it's all pretty early-stage stuff.
Until the sandboxing gets better, you should be cautious about installing unvetted desktop and phone apps. You should also be cautious about installing unvetted browser extensions. But browser extensions are complicated because while keeping a minimal system isn't that hard, you're probably not going to stop visiting unvetted websites, even if you know it's dangerous. It's a much higher priority for experienced users to make the browser sandbox good than it is to make the extension sandbox good.
People take a long-term view on this, and while I agree with them in theory, I don't think it's always particularly helpful to think about what technology will look like. With browsers, it's not a question of whether or not theoretically it would be good in the future to make extensions entirely declarative. Of course it would be good. It's a question of, 'is it possible to do that right now?' At the moment, Safari's declarative API is significantly less powerful than the blocking API that Firefox has. In the future, that could definitely change, but people have to use computers today.
So for the moment, the browser advice I give to non-power users is to install UBlock Origin and Decentraleyes on Firefox and nothing else. I think that's a safer, more private environment than anything they'll be able to set up on Safari. I advise power users to add uMatrix to that list, and for people who are really paranoid, I advise them to run Firefox Developer edition, which will let them compile extensions from source.
If you're just handing someone a computer and you don't trust them not to go off and install random extensions, then sure, give them Safari. In that context, it's not confusing why Apple would do this -- they're optimizing for the largest number of users; people they can't trust not to install random extensions. It just means that more experienced/responsible users will be safer using Firefox.