But what you're suggesting is more of what ought to happen as an internal mechanism of US politics - the important issue here is that regardless of how often we hold national referendums on the electorate's opinions of various treaties and agreements, we need to abide by those agreements while they are legally considered "the law of the land," which they are the moment they are both signed and ratified.
On another note, from whence did this notion come that the general population should be deciding on matters of foreign policy, i.e. treaties? The idea of government by the best & brightest scares me as much as it does anyone else (particularly because it's so painfully clear that we LACK this right now). But, the general public surely has neither the sophistication nor the expertise nor the time necessary to learn about the multitutde of treaties of which the U.S. is a signatory. And, even when they DO have opinions, they are commonly over-ruled by a zealous executive branch. Most Americans support both the Kyoto Protocol and the Anti-Ballistic Missile Treaty, and Bush II gave both of those the boot.
This segues directly into why exactly the Constitution has stood the test of time - not necessarily because it sets out the world's closest-to-perfect system of government, but because the vast, VAST majority of people are embarassingly apolitical - apathetic and ignorant, even. A robust and thriving democracy isn't possible in the face of such a populus... most of them don't even vote, let alone have much to say about foreign policy treaty decisions or Constitutional restructuring.