Agreed, you cannot "lock down" individual states from one another. As for quarantining, this may work on an individual level, but not on a massive public level. Forced quarantines are always breached, which is why social distancing is so much more efficient, as the simulations here show:
https://www.washingtonpost.com/graphics/2020/world/corona-simulator/
This also shows why travel bans don't work, especially when their enforcement has been so lax and has been full of loopholes. A travel ban may buy you one, at the most two, weeks time, but the end result will be the same.
Why don't travels bans work? Why couldn't individual states restrict movement across their own borders with the exception of trans state commerce? We have all read reports where RI is restricting NY residents and our own Cape Cod, either residents or local leaders, wanted to close itself off from the rest of the state. Small coastal communities in ME want to keep non residents out, including those from NY who own houses there. Heck, the next town over from me even restricted use of their beaches to non town residents.
If this gets much worse, I can see a scenario where it was countries acting in their own interests first, then states, then cities, then towns, then neighborhoods, then individuals. I hope it never reaches that point, but I saw hints of this kind of response a couple of weeks ago.
We are a country made up of a federation of individual states. There are boundaries between the Federal Govt and state governments. I'm no expert, but why is it necessarily the case that only a complete country wide solution by Federal fiat is the only solution that many people consider viable? We have already seen governors acting on their own, and they have been praised for their initiatives. The US is a large, incredibly culturally diverse country, with huge differences by region, state, and county.
There are some instances where a strong federal response is warranted, and there are others were a more nuanced approach may be most effective. One example may be a federal aviation ban for citizens within the country and a ban for all non citizens at our national borders. But within states, or regions, it is not clear to me. Doctors on TV are saying that if there are no people on the hiking trail with you, take off your mask, if there are people, keep the mask on. This recommendation is a direct response to different conditions on the ground.
I don't see why the US is so frequently compared to much smaller, more homogenous countries and cultures. Customs, needs, populations, behaviors, and conditions on the ground are often very different. Such simple analogies are not so simple, IMO.