There are many HDD's that have more than one plate to sotre data... Thinking about a simple HDD with two plates... Why can't we use one plate as one HDD and the other plate as another HDD (as in RAID)?? Just add a RAID chip and other disk controller in the same package and then we would have RAID in a singles HDD... Why nobody ever did that??
This in theory would work. But the real problem is... the processor for the RAID.
On a dual disk RAID system, the data is processed either by the CPU for onboard RAID or by a dedicated processor on a RAID card. At this point, it is easy to take one chunk of data, split it and send it to the controller, which in turn, send it to the coresponding disk. If you want to tun RAID with a single disk, then the data chunk had to be sent in whole to the controller, then to the drive, and then, splitted and sent to the right platter...
Now, you need on the HDD, a complete RAID processor, maybe some buffer memory and the electronic to drive all that. This would create an expensive hdd, which would draw a lot of power, and generate a lot of heat.
With onboard RAID solution that does a decent job with the help of the CPU or a dedicated RAID card with its dedicated processor, that cost 200$ or more, with support for 4 HDD, then it would not be wise to get 4 hdd that would cost 4 time as much for the same storage capacity. The interface can take it, the hdd mechanism could drive it, but the electronics cannot simply made effective and cheap enough to worth it.