So many responses here, but mostly: "Most software doesn't run with full system privileges", "For most people, yes it does."

That's not even a little true. Even inexperienced Windows users have to click that little "permission to make changes" button, and sometimes "run as administrator". Winamp, Solitaire, and all your other random not-totally-secure programs don't offer kernel access to whoever compromises them. AV is a singularly good source of privilege escalation attacks for exactly that reason.