That wouldn't work. Even TPUv4 was on a 7nm node and you don't just build a 7nm fab just like that. If it were that easy NVIDIA would already be building their own fabs, as they have basically raised as much capital as there is in the known universe (bigger market cap than the entire London stock exchange), but they seem to prefer to let the fab experts get on with it rather than compete with them.
LLM AI is largely HBM bottlenecked anyway i.e. Samsung, SK Hynix and Micron are where the supply chain limits enter the picture.