My theory is demand substitution. NVME/SSD is up so much people at the margin are switching to spinning rust, which nobody was prepared for, so now they too are drowning in demand.

(1) Because they can. Implied

(2) If you're WD, Seagate, etc. you may shift all your fabs to higher growth areas which means you create less supply for less demand products (spinning disks)

I've resorted to buying refurb HDD's for my NAS (which I backup to rsync.net) but like damn!