Such a great point. They shouldn't.   Diplomacy in the American political system is frequently described as the exclusive province of presidents. Thomas Jefferson, America’s first secretary of state, wrote in 1790, “The transaction of business with foreign nations is executive altogether. . . . Exceptions are to be strictly construed.” A decade later, John Marshall, who would go on to become the most influential chief justice in U.S. history, declared on the floor of the House of Representatives, “The president is the sole organ of the nation in its external relations, and its sole representative with foreign nations.” Justice George Sutherland noted Marshall’s claim in United States v. Curtiss-Wright Export Corporation (1936), perhaps the most frequently cited case dealing with foreign affairs, concluding that the president “alone negotiates: Into the field of negotiation, the Senate cannot intrude; and Congress itself is powerless to invade it.”  source Backseat Driving: The Role of Congress in American Diplomacy