This is primarily intended for the DoD/CIA/NSA
That's the only explanation for making a fab that will be four years out of date when it is completed, and nowhere near the size of TSMC's Taiwan fabs at only 20K wafers per month. For scale, if that fab was solely dedicated to making Apple's iPhone/iPad SoCs and it yielded at 100%, it would meet less than 75% of Apple's yearly needs. TSMC's leading edge fabs serve not only Apple but also Qualcomm, Huawei, AMD, Nvidia, and so forth, and that's where all of them will continue having their chips made even after this fab is up and running because they will want to use TSMC's leading edge, not a by then two nodes out of date 5nm process.
I think the US government wants dual sourcing and now that the former IBM fab (now owned by Global Foundries) in upstate New York is becoming more and more obsolete and won't be upgraded, they need another source aside from Intel.