It costs billions, and takes years to bring up the sort of manufacturing plant and infrastructure needed to produce silicon. It is happening, but the investment and expertise needed to do it means it’s limited to a handful of countries that are realistically capable of doing it. There’s also a huge risk of he investment not paying off in doing this too, most of the worlds high tech manufacturing is based in the Far East too, plonking a silicon fab in somewhere like North America makes it more expensive due to labour rates and the cost of shipping chips to the point they’re assembled into pcbs