Because /usr may very well be on a separate file system, and the root account needs to function even when all other file systems (other than the root file system) are not mounted.
(Side remark: I've never liked the convention that regular users home directories are in /usr, and that /home is just a symlink. I always fix that on my systems by making /home a really large separate file system. Like that, there are only two file systems that regular users can scribble on: /tmp and /home.