The picture I linked is of a buckwheat pillow. Had no idea they were a thing before today.
What about silk or bamboo pillow cases?
I've only ever really used cotton pillow cases and cotton stuffed pillows. I recently upgraded to a foam pillow that I'm enjoying.
Probably the same reason waterbeds fell out of style.
They're more trouble than they're worth.
I understand the issues with waterbeds, but for pillows I really don't see the trouble: You don't have to heat them or otherwise maintain them, except changing the water every few years, which is done in 3 minutes: unscrew, old water in the sink, new water in from the tap, close it up again.
Ngl I know nothing about water pillows I'm just using conjecture from having used a water bed.
If you have pets of any kind it's inevitable that somethings gonna pop.
The issue with either is from the risk of popping/leaking: if your pillow leaks, your entire mattress might be fucked. The risk vastly outweighs the improvement