A couple of strategic reasons: Taiwan's proximity to the PRC would knock out fab production should a war occur between the two. Also, the same proximity could l

A couple of strategic reasons: Taiwan's proximity to the PRC would knock out fab production should a war occur between the two. Also, the same proximity could lead to unwanted IP transfer, which could compromise US national security if China gets its hands on any new chip tech the US develops. Taiwan is a great ally to the US and having it manufacture chips is good for both economies, but if the US wants to lower risk and keep control of its technological edge it is in its interests to keep manufacturing domestic.