Also: how many laboratories working with novel coronaviruses are there in the world? How concentrated are they?
I don’t know the answer, but it could well be that a significant fraction of the population of China (or the USA) lives in a city with such a lab, making it not too unlikely that the first patient will be found in such a city.
Wikipedia has a list of all the biosafety level 4 facilities.
The answer is around 30 active sites that deal with threats as serious as the Wuhan facility. Of those, less than half work on viruses, and even fewer do GOF work.
I don’t know the answer, but it could well be that a significant fraction of the population of China (or the USA) lives in a city with such a lab, making it not too unlikely that the first patient will be found in such a city.