Maybe because the physical chip itself has not been verified to operate correctly under various noisy conditions (x-rays, magnetic fields, cosmic ray errors...). E.g. launch computers like this [1] would use redundant circuits with majority voting, and even redundant voting logic.
Perhaps. I'm surprised that article doesn't mention radiation. You're right of course that radiation-hardened chips are a very specialist area. Not often you can charge hundreds of thousands of dollars for a single CPU chip running at 200MHz or less.
I'm really curious about the processes used to make these. Part of it is because it uses huge features, of course, but I wonder if there are other things to it such as the materials and the encapsulation.
My guess is that "life-support systems" is what immediately came to mind when their legal council was contemplating potential realms of excessive liability.
[1] https://en.wikipedia.org/wiki/Launch_Vehicle_Digital_Compute...