Because India is also putting a lot of effort into submodule production. It's one thing to do final assembly in a location. What India is doing is putting a lot of effort into building the ancillary industries that accompany final assembly. All those discrete resistors and capacitors that go onto the PCB have to be sourced from somewhere. If you run out the line can't do anything. This doesn't happen in cities like Taipei, Guangzhou, and Shenzhen because there's literally infinite amounts of these commodities in the area thanks to all the manufacturers of these commodities.

Nobody in the US makes SMD parts anymore. It's all higher up in the value chain. But those parts, even if they're commodities, still need to be made. India is going for it which is why Apple is diversifying there instead of the US.