They could, but the answer to "what should I spend $20b on" for both pretty clearly isn't "a fab where at the end of the day you might get a fraction of a percent better costs on bare dies from the vertical integration versus just buying from TSMC or whoever"

Fabs are both a commodity, and take a high capital investment. Unless you have a geopolitical reason to create one and you get .gov kickbacks to make it happen, then there's way better uses of your money.