Even on a single user machine, multiple users are very much a thing. Even Apple has left behind the DOS-like architecture where everything runs with the same rights. All current systems run with multiple concurrent users, notably root (or the Windows equivalent) and the keyboard operator (as well as dedicated ones for the various services, although that's maybe more a thing in Unix/Linux than Windows).

Good point. But I think performance is still a greater priority for those who make purchasing decisions, rather than basic security, and that's the problem.