From wiki: "A typical magnetic region on a hard-disk platter (as of 2006) is about 200–250 nanometers wide (in the radial direction of the platter) and extends about 25–30 nanometers in the down-track direction". It's not trivial to say "Let's get X and miniaturize it to this scale".

Flash memory uses standard semiconductor techniques, and HDDs simply employ platters with tightly controlled properties.