"What's stopping them from short stroking disks that they want fast I/O from?"
Why would you bother when SSDs do this fantastically better?
The reasons for not having multiple sets of arms have already been discussed.
Short version: It didn't work. Slightly longer one: When it did work it was unreliable.
The reasons for not shortstroking are simply that you end up with a device that costs about as much per GB as a SSD (or more!) with performance far inferior to SSD (which is why SSD has eaten the 10-20krpm market too)
If the chocolate factory put in an order for a couple million devices they can get them customised any way they want, but...
1: They won't be cheaper than commodity items (they'd need to order 10-100 times that many)
2: HDDs aren't (practically) going to get any denser. All the R&D labs are closed. HAMR is end of the line and the extra head size for that has proved to outweigh the platter density gains.
3: All the platters come from just two manufacturers (possibly down to one now) and all the heads come from another manufacturer. Both of them have been coordinating years in advance with drivemakers to ensure economies of scale in manufacturing. The Thai floods were the warning shot that HDD supply chains are subject to nasty SPOF issues. BASF is not going to crank up a 5.25" platter line just because Google wants a couple of million drives, etc.
4: SSDs are still getting denser/faster. 3.5" and 2.5" formats are legacy items now, just as 5.25" and 8" were in the past.
Bigfoots were fun, but slow, fragile and even if treated with kid gloves, pretty unreliable. Having arms that had to seek over that much platter space and cope with that much variation in resultant linear velocity under the heads (which affects fly height, because it's that velocity which drives air through the venturi that lifts the head off the disk) turned out to be a bad idea at those platter densities and wouldn't be possible with current platter densities (which have a fly height less than 1/10 of bigfoot-era heads).
It was rumoured that Quantum tried multiplatter bigfoots but that they never even got to production prototype stage due to unreliability.