It was also arguably “unconstitutional” [1] for Australian states to close their borders with one another over the pandemic. Business magnate Clive Palmer sued Western Australia over its border closures, in fact, and even received backing from the Australian federal government [2].
Particularly in light of the patchwork framework surrounding state-mandated church closures in the US, the difference between Australia and the United States as it pertains to interstate border closures seems to be more a matter of cultural expectation, than legal impossibility. But IANAL.
US airports are highly controllable ports of entry. I think a lot more could’ve been done there had the population willed hard border closures into existence.
[1]: https://auspublaw.org/2020/08/border-closures-and-s-92-clive...
[2]: https://www.theguardian.com/australia-news/2020/aug/02/feder...