All courts are theaters, some are backed up with an army.
The president of the United States just proved that the supreme Court has no power by refusing to bring Kilmar Abrego home. The people saw this happen and didn't immediately take to the streets demanding that Trump be punished, so we now know that he can do whatever he wants and the supreme Court can't stop him. There's no difference between this and the ICC, which is one of the ways that the United Nations prosecutes criminals. Both are theaters of Justice that only work if people participate.
The fact that America isn't involved in the ICC really shouldn't be seen as the ICC not being legitimate, but instead that the United States isn't.
Which one, you just described 4-5 nations