Because it has no jurisdiction over Americans. Although Clinton signed on to the Rome Statute establishing the ICC on the last possible day, after 18 months of saying he wouldn’t, the court at the time did not have enough countries ratifying (60 were required) to go into effect, even had the US Senate approved it p, which they never did. In 2002 President Bush withdrew presidential approval before the 60 country ratification requirement was ever met so there it sits ever since. Nobody can legally extradite any US citizen to the court under US law, making it essentially a kangaroo court for US legal purposes.