Ok, you've got me
I failed to take into account that the disk is already spinning for most data access. I had visions of an array of heads arranged like a scanner bulb, moving up and down a platter.
It's not my idea, but I remember a few years ago (when SSD technology was still future tech) there was some development into a a stop-gap solution for seek time and RW speeds.
The plan was to make the hard drive platter square instead of round and have a plate of read/write heads over it. Each read head was only ever a couple of mil away from any sector it had responsibility for.
The idea was that the plate would only have to move that couple of mil in two dimensions, drastically reducing seek times and the read heads would read and write in parallel, improving read/write speeds.
I'd imagine it would be a noisy bugger though. Still, you wouldn't have to worry so much about wear leveling and difficulties with secure deletion. It's a shame it didn't take off.