Most desktop OSes came about (or at least have their roots in) a pre-internet world, where you install software from discs you purchased at the store, or if you

Most desktop OSes came about (or at least have their roots in) a pre-internet world, where you install software from discs you purchased at the store, or if you're feeling gutsy, from media your friend hands you in real-life. They assume you have a great amount of trust in every piece of code you run on your computer (and anyway, how will malware exfiltrate your data without an always-on network connection?). Things like Windows Defender and Snap and the recent macOS hardening efforts are patchwork solutions to try and cope with the modern world, but they'll never really be enough because these systems can't be fundamentally re-thought; they have to keep doing everything everybody already expects them to do. Only brand new OSes really get the chance to do things right, and only the mobile ones really had the opportunity to gain wide adoption.