It's quite literally not. Root is technically a user with extra rights (including modifying the kernel, but there is still an API the root user has to go through). This is running as part of the kernel. It's not running in userland "as root".

A rootkit is something that gives other users the power of root.

That's taking it too far. By that logic, rootkits can't exist for Windows because the super user is called "Administrator", not "root".