[avianlyric]

Cost, more resolution means more transistors, address lines, and protocol overhead (your memory address take up space too).

You could build a HDD with byte-wise resolution, which did a whole load of clever things internally to handle error correction (modern HDDs have far more complex error correction that just block wise checksums, they’re a piece of technological magic and analogue electronics).

But sure a device would cost far more, and offer no real benefits. Especially when you consider that reading a single byte in isolation is a very rare task, and it takes so long (compared to accessing a CPU cache) to find the data and transfer it that you might as well grab a whole load of bytes at the same time.

Byte-wise resolution in a HDD is like ordering rice by the grain in individual envelopes. Sure you could do it, but why the hell would you?