> why isn't the west's own supply chain options as immense?

Because each city in China has become specialized. You want to have someone make hairdryers for your company to sell? Then go to Cixi. There are dozens of small suppliers making the parts that go into hair dryers. There are dozens of companies making small appliances (just like hairdryers) They're all "just down the street" from each other. This means that the knowledge and infrastructure and workers are all in one place. You don't have to ship a truckload of heater elements across the country to some factory that some CEO decided should be built in the lowest cost real estate. The same reason that all of the America IC manufacturers got started in Silicon Valley.

This sort of specialization/concentration used to happen in the US. That's why NYC had a "garment district" where you could get clothing made from design to ready-to-sell. Los Angeles used to be one of the major hubs for making aircraft because of the large number of small companies making stuff that the aerospace companies assemble into aircraft. Jacobs wrote about this sort of thing in Cities And The Wealth of Nations about how the Shah of Iran wanted a helicopter factory in Iran. It was a flop because none of the seats are made across town, they're made in America, like the blades or engine or windscreen or avionics. All the Shah got for his dream was an assembly plant. There was no transfer of technology so that the parts could be made in Iran.

Before shipping containers were invented, shipping goods was expensive enough that factories making things tended to be located close to their suppliers. That was why Detroit became a center of car manufacturing. Shipping containers made it cheaper to transport some item across an ocean than it costs to drive it across the city.