It takes years just to figure out how to use the machines to produce actual working chips. They're the most specialized, intricate, expensive machines in the world. They're operated in enormous clean rooms that only allow for 1 half-micron sized particle per cubic foot. No fab is going to run them without exactly following the procedures they have spent billions developing and testing. They are also going to avoid potential delays as much as possible because these machines have to be running for as much of their useful life as possible to recoup the vast expense. The level of risk-averseness is insane, but warranted.

If you haven't watched this video, I highly recommend it: Indistinguishable From Magic: Manufacturing Modern Computer Chips https://www.youtube.com/watch?v=NGFhc8R_uO4 It's a little outdated now, but very comprehensive and gives you an idea of how totally nuts the chip business is.