Because the US conveniently aren't part of the treaty.

The U.S. signed the Rome Statute in 2000 under President Bill Clinton but did not ratify it.

In 2002, President George W. Bush's administration formally "unsigned" the treaty and passed the American Servicemembers' Protection Act (ASPA), which restricts U.S. cooperation with the ICC and even authorizes measures to prevent the prosecution of U.S. personnel.