It seems like an odd situation but politics are never logical at times. The whole thing was just a theoretical exercise in military planning. The brass would have made better use of their resources planning for a war with Germany, but that wasn't politically expedient. They reasoned that planning for unlikely wars was better than no planning at all. War Plan Red was never intended to be put into action except in the event of a war with the United Kingdom, an eventuality that everyone would agree was highly unlikely after about 1900.
War Plan Red was one of many "color plans" developed in a fairly systematic way in the 1920s and '30s. Except for "Orange" (war with Japan), the plans were primarily academic exercises, lacking detail and offering only broad outlines of strategy. The plans were an outgrowth of military reorganizations that had led to the creation of the U.S. Army War College (1903) and the U.S. Army War Plans Division (1921). Planning capability having been established, the military figured its planners had better get in some practice. The result was a dramatic increase in formal planning for various military contingencies, most of them unlikely.
Lets face it, America was built on war & conflict, abroad and at home. Does this come as a suprise to anyone?. Better to be prepared than not...