Fault isolation. We're doing it for daemons, we're doing it for web browsers, it is insane we're not doing it for operating system services. I bought a graphic tablet and the first time I plugged it into my laptop the Linux kernel crashed. And this was merely a faulty driver, not even malicious hardware.

Also think of the effort it took to introduce namespaces to all the Linux subsystems. After a decade the user namespace still has problems. This is ridiculously easy on a distributed system, yet very hard on a monolithic one.