Shenzhen can also produce that supply chain, but they are no more capable than a lot of American cities.
What Shenzhen has that America doesn't are cheap labor, and a devalued currency.
Silicon Valley is different primarily because there is no other city with the concentration of brilliant engineers vs others. NYC for example probably has more computer engineers by numbers, but there are more opportunities for those engineers to make huge pay in other industries outside of startups. The fact that Silicon Valley is full of amazing engineers who are excited to work in the tech industry means that it is the best location for your startup to locate (relatively, and if you need a lot of engineers, not all startups do, some need more sales focused or science focus etc and there are better cities for those companies).