I suspect the difference in demand is huge. I still have stacks of 3.5" disks sitting around the house. I haven't had a 5.25" inch disk sitting around since the early 90s.
I'm about the same here. I should really order a USB 3.5" drive to get the last data off them this year.
I got one about a decade ago as a just in case... it's in my garage somewhere, still in the box.
Doesn't the magnetic data on the disks fade over that time scale?
Dunno... I don't actually have any 3.5" media... but got one because back then, I was more inclined to help people with things.
Perhaps the missing piece of information is that: nobody makes any of these drives any more. The 3.5 drives sold are either old stock or refurbished. Heck I bought my last one (to make bootable floppy images for some patent litigation project) probably 10 years ago and that one was a refurb.
Or _not_ refurbished, according to what I recall reading on some vintage-computer discussion — the typical Amazon-grade USB floppy drives contain scrap pulls that may or may not still work.