Economics
Perhaps somebody more informed about the fab business can answer this.
A fab costs $MAX_INT to build - so you need to run your declining asset at 100% and presumably raw materials aren't a huge cost.
Similarly the high cost staff are in the design/tuning/process control end, not zero hours contract people you can scale down.
If you run the fab you know industry trends for how long the current DRAM/NAND design will be saleable before it's obsolete.
So in a downturn like 2020 wouldn't you run at some capacity making parts for stock, even if not at 3 shifts + overtime?
At least if you are a Hynix or Micron who own the fab rather than a TSMC making other people's stuff.