Why the US still doesn’t have a contact tracing app for Covid-19

Contact tracking is probably useful at the beginning of a new outbreak when the disease is still isolated to a small number of locations. Isolating a few people might prevent the disease from spreading to other areas.

As you described, once an outbreak has a foothold in most areas, most people will be within one or two hops from someone who is infected. Also, the potential useful period will be shorter when the incubation period is long. Two weeks of incubation provides a lot of opportunity for someone to spread the infection.

Ya, I see what you mean. Countries like South Korea and studies from Wuhan seem to indicate there is real value to doing contact tracing though, even without an app. Check this one out from the essay: https://science.sciencemag.org/content/early/2020/04/09/scie...

And Google “Patient 31 South Korea” to get a nice visual of the problem

You can modify the algorithm to do n-degree contacts as well though and then you can propagate a positive test through the network accounting for time delays. Contact tracing should allow you to be more effective with fewer tests since you can ensure that people who wouldn't normally self-isolate do so.