Fabs do not work on jit/lean order principle - if you up your order volume, you can't simply scale it up on demand the next month - even if your design is done and taped out. Fabs aren't sitting around idly waiting for orders nor are they overprovisioned "just in case".
An example of this would be AMD and TSMC. AMD books capacity 3 years in advance for which TSMC designs the entire fabrication facility around, orders the lithography machines (which have a 12 month lead time and can't scale up on a dime) and assembles and staffs those same facilities to produce the wafers. It's not a general-purpose fab as most think. There are rumors that Apple booked m1 fab capacity (on TSMC 5nm) in 2018. For the chip inside a car for example, it is built on much older process, and when 5 of your biggest customers "JIT" reduce their order volume for the next 3 months you simply put those staff elsewhere (probably on the 6x higher margin cutting edge fab you just got booked and prepaid for) since anyone else who might want that process and capacity has already has their orders being produced.
Due to the sheer capital requirements (and consequent commitments required) and multi-year forecasting involved in simply getting what you think you'll need, let alone having extra, it's not hard to see why a small increase in global demand would cause a dramatic shortage. JIT works in Automotive supply chain where you can request that your bearings get made 10 days before they're delivered to the factory, but not when there is an 18 month latency on "scale up" request/response.
It's applying JIT principles to inherently non-JIT supply chains.