It's because ZFS is much more than RAID. RAID just exposes a disk to the OS. ZFS exposes a filesystem and knows more metadata, e.g. the checksum, so it knows when to repair the data using the redundancy from the disks.
I think this is a good introduction to PID for people who don't want to have to dive into the nitty gritty. Linear feedback controls is a complex but complete theory and it would take at least a semester at a university to understand the theory behind time and frequency domain design and the effects of discretization.