A, the USA is fuckhuge and cycling culture is not as large here with the exception of select areas on the coasts
B, to actually go cross country, the entire center of the country is 2,000 miles of flat time trial land
C, even in the winter, people would be fucking dying in the low desert
D, Americans don't deal well with closed roads plus civilians would be obnoxious wherever there is an opportunity
That said, I'm planning a trip at the end of my PhD to go from the Washington Monument to the end of 66 in L.A. But I'm not racing.
The US lends itself extremely well to car racing. For most other things, you're still going to be suffering through 2,000 miles of the most boring shit you could possibly imagine. Drive from Emporia, VA to Savanna, GA. You will feel like you've just driven the same five mile stretch of road, just 200 times over.
If I was in charge of planning road cycling races in the US, I'd really do something like "The Appalachian Sprint," "Head of the Rockies," or "Escape from NY." Something where you can actually have comparable numbers of race, TT, and mountain stages.