We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
There are a lot of factors. Manufacturing process, head, lubrication, insulation, material thickness.
Nobody can give you a specific answer without a forensic teardown. I would say it's probably the casing, refinements in head size, and noise canceling insulation inside the drive.
Drive noise measurement is always a frustrating thing to keep track of. It's much easier to just find a solution to isolate the drive noise completely and not worry about it.
Helium