I suspect the head flying just above the surface has something to do with it.
A bar would need to be held at a fixed height above the surface. The flying head dynamically adjusts to both distortions across the surface, and temporal variations as the platter changes temperature. This height adjustment is entirely passive, dependant on aerodynamics rather than any electronics.
I remember someone explaining this to me years ago when IOmega released those removable Bernoulli drive cartridges. The (for the time) high densities were possible because even though the recording medium was a flexible bit of plastic, the head was designed to draw surface of the media up to itself using the namesake effect, rather than relying entirely om precision machine tolerances. And this effect was dynamically self limiting, such that there could not be a head crash as long is the unit wasnt disturbed during operation.
What I did not know is that this is still and effect that is relied upon in modern, ultra high precision, fixed medium drives.
A bar would need to be held at a fixed height above the surface. The flying head dynamically adjusts to both distortions across the surface, and temporal variations as the platter changes temperature. This height adjustment is entirely passive, dependant on aerodynamics rather than any electronics.