Because every treaty we sign with them is constitutionally binding. General Agreement on Tariffs and Trade regulates how we can trade, NATO was used by us to lock other nations into Operation Eagle Assist, several defense based treaties force us to defend others, and likewise, whether it is beneficial for any party concerned, or not. Open Skies treaty is an interesting one, since it forces us to allow other signers of the treaty to monitor our military forces and activity via aerial recon. There's more, but I'm tired and have to start winding down for the night.
The bills are the problem. Our obligations to the UN through these treaties are costing us more money than we can generate.
Treaties have a lot of authority. Article VI of the US Constitution states:
"This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding."