They could also be staggered in tiers, one row offset from the other.
I feel like there's an alternate universe where this is exactly the design that was adopted and someone is out there suggesting our universe's setup instead, asking why not make a single read-write head that gets whipped back and forth around at mind numbing speeds seeking to different spots on the surface of the platter, and everyone's pointing out how absurdly bad and pointless an idea that would be: think about how delicate such a setup is, you want to put a flying bit of metal over a rotating metallic surface at just a microscopic distance apart - you want to rely on the Bernoulli effect to pull it into just the right proximity to the surface, and this has to work in every possible temperature, vibration, and moisture and air pressure scenario? how do you compensate for inevitable wear and tear on the mechanism in the drift and loss of precision that will induce, what about all of the fine particulates that may develop from friction over time, how do you get lubricants to last so long, all the possibilities for terminal head crashes, how slow mechanical mechanisms are versus solid state, the comparatively massive additional power draw and heat this would introduce, and so on.
Without knowing anything about the specifics of material engineering whatsoever in this domain, my assumption is that Manufacturing the read-write heads maybe one of the most expensive technical challenges and perhaps a disproportionately large factor in the cost of the entire assembly, so perhaps asking for an entire strip of these things might be like ignorantly asking, if the light-refracting feature of a diamond is the most salient feature of a diamond ring, why not make a rings with diamonds all the way around instead of just one.
Something similar could work with hard drives: an array of about 3 to 4 thin strips of silicon chips with a line of read/write heads.