> In my view, the real question is why others are so willing to give up their sovereignty and right to self determination?
Because in doing so in the form of an agreement, others can be made to give up their freedom to do bad things while you also give up the freedom to do those bad things? Like the normal social contract, but explicit and on a larger scale.
This dates even to the 1899 Hague Convention, which the US was very keen on following earlier work by Abraham Lincoln. The way to get the enemy to treat your prisoners of war humanely is to agree to treat theirs humanely, and this held fairly well even in the horrors of WW2.
The post-Holocaust international order is devoted to the idea that there is always a risk of rogue states deciding to end the freedoms of their citizens or engage in mass murder and other crimes against humanity. There are also risks of states turning on one another and escalating economic disputes to actual wars. In order to guard against these, supranational institutions were built. Not just the United Nations and the ICC, but the whole Bretton Woods system, the WTO, the NATO mutual defence organisation, the ECHR, and the EU and its antecedants.
How can the US ask China to end its human rights abuses, and expect to be taken seriously, when the US refuses internal or external accountability for its own human rights abuses and war criminals? The official position of the United States, having determined that Edward Gallagher was deliberately murdering civilians and prisoners of war, is that he should be pardoned and have his position reinstated? That's fine and no reason for imposing sanctions on the United States?
Yes, this is a tu quoque argument, but at the level of international propaganda it works.