I think it's also because a desktop is broadly considered a "power tool", you absolutely need it for systems development, and that is forever at odds with "iPhone-level" lockdown/security. Even if you'd start from scratch, you'd need a solid theoretical foundation (e.g. capabilities), and probably build up new metaphors on top.

I like what OpenBSD did with pledge&unveil. It gets "the first 80%" of the work done, it's easy for the developer, and invisible to the user.