What need is there to centralize those resources?

The inability of any one state to marshal those resources as effectively as the central government can, not to mention the supply chain debacle of the central government interfering with the acquisition of those resources.

That seems to contradict the empirical outcome of COVID, where smaller nation-states (Denmark, Singapore, Taiwan, New Zealand, etc) were able to provide for their citizens much more adequately than larger counterparts.

I recommend listening to this podcast that outlines exactly why smaller polities were able to be nimbler: https://soundcloud.com/reuters/the-exchange-too-small-to-fai...

Yes, economies of scale provide advantages for procurement, but after a while, there are diminishing returns from more scale. That is to say, New Zealand's population is a sufficient scale, it need not pool its procurement operations with Australia. It's a key differentiator between Federalism and Individualism.

That’s an artificial limitation, not helped by the National government’s penchant for seizing State orders and stockpiles under the current administration.

Without crap like that, there’s no reason California or New York couldn’t create their own medical stockpiles. They could run their own testing regime at least as effectively as the Federal government, but the FDA actually hindered testing both by the States and private companies early on due to an old regulation based in an old law that they could not shed fast enough. States were also quick to shed regulatory barriers of their own making, which should probably remain shed after this is all over.

I guess the idea that all 50 states can replicate this effort effectively is something you can believe.

You don’t need all 50 to do it. Wyoming’s largest metro area isn’t that far from Colorado’s, and two of Missouri’s largest cities are on its East and West borders.

States stockpiling or relying on Federal stockpiles or making agreements to share stockpiles is as much a choice as any other. Seeing as how the Federal stockpile was not properly maintained and refilled by either the Obama administration or the Trump administration (who could have at least checked on it) after H1N1, maybe at least one of them would manage to do a competent job of it.