Or, to put it another way, supply chains. Our current technology was built up by making stuff to make stuff to make stuff to make stuff etc. If we suddenly lost access to modern factories, we might be able to hold on to late 19th/early 20th century (~WWI-era) tech. Move much past that, and you can no longer bootstrap yourself up in a reasonable time by starting with a talented blacksmith, a pole lathe, and some gumption—you need complex machinery, an electrical grid, and so on. (In the case of the Saturn V, we’ve lost just the last couple of layers of stuff-to-make-stuff. We could probably recover it if we really, really needed to, but it would be prohibitively expensive, and that’s without considering environmental controls and the like.)

It’s remarkable how much small stuff is vital and nearly impossible to make in reasonable amounts without a mechanized process (and perishable enough that you can’t keep them on shelves forever). Things like ball bearings and transistors. We don’t think about them much, but they’re vital for (respectively) modern mechanical and electronic technologies.

Plus, Japan isn’t exactly the most mineral-resource-rich country. They have little oil, no aluminum ore, only small amounts of iron and copper, and what’s left of their coal apparently isn’t of the best quality. It’s difficult to maintain much technology without at least some resources.

They’ve maybe regressed a little further than strictly necessary in this show, but not by much.

(What bothers me more is that they have food supply issues, and yet they apparently reserve a fair amount of land for growing . . . tobacco. Not exactly the smartest decision.)