That's because they intervened early with large scale testing, contact tracing, and social distancing.
The problem is contact tracing only works when the number of new infections is manageable, though. That's why it's so critical to react early and aggressively. Once it becomes unmanageable, the only solution is mass quarantines until the numbers are manageable again.
The best analogy is a fire. If it's just embers, one person can stomp it out. If you wait for it to become a forest fire, then it requires drastic measures.
Clearly we (in the US) need quarantine now when things are out of control (and we need adequate PPE equipment for those working and financial support for those not working, two things that seem to be failing, likely to our extreme detriment).
But once the "fire" has burned far enough, we will testing and contact tracing. And so we need to be getting these up and running NOW so they will be ready when the "fire" has subsided sufficiently (and hey, we are failing on this too).
There doesn't seem to be much of a plan in place to test and contact trace in the USA. As you said we need to be getting these up in place now. And they should already be in place in areas without a lockdown.
But I don't see much evidence we are really trying to do that.
I also wonder if its possible to test and trace if its every minor city in the country in low numbers. We will have a million patient zeros.
The problem is contact tracing only works when the number of new infections is manageable, though. That's why it's so critical to react early and aggressively. Once it becomes unmanageable, the only solution is mass quarantines until the numbers are manageable again.
The best analogy is a fire. If it's just embers, one person can stomp it out. If you wait for it to become a forest fire, then it requires drastic measures.