Why can't they keep things going temporarily by spinning up those n-th tier fab wannabes? (your words lol)
In my estimation, there are few trustworthy foundries that can facilitate the kind of scale that an automaker would need. If you're in a hotly competitive industry, you're not likely to contact some random fab freelancer to do a few runs of some chip you need, even if you have the design and everything ready to go. They're unlikely to do it at the quantity you need, on the timeline you need, to the degree of quality that you need, and with the secrecy that you need.
I mean, if you're Denso or Bosch, shouldn't you be able to make your own chips? Auto parts have a long life cycle, so you can amortize the cost of the machinery over many years.