Same reason most people prefer living in a society with laws: you are subject to laws, but so is everyone else, and provided the laws are beneficial ("just") you are overall better off.
On a national level I agree not to steal, and in return nobody else is allowed to steal from me. On the ICC level my country agrees not to genocide anyone, and in return others aren't allowed to genocide either
The fundamental problem with this logic is that there is no enforcement body here. In a just society, the law defines who is "allowed" to steal from you and the state's enforcement arm punishes those who violate the law. The law binds but also protects.
Creating a global enforcement arm is an obvious non-starter: Which laws shall be enacted and enforced against whom, and how are fundamentally political questions. The answers to those questions necessarily change once you cross into another polity. States can coordinate and agree mutually to enforce a law... but if Poland says that despite their commitments to the contrary they simply aren't gonna extradite that suspected Nord Stream saboteur: Tough nuggets. Such situations expose international "laws" as mere window dressing on powers that emanate from and remain held by individual states.
There is no global enforcement arm and plenty of evidence that if someone starts genociding you, nobody else is gonna do anything about it. You're on your own. Such "laws" bind but do not protect.
Especially in the context of the US as world hegemon: The US fully expects to shoulder the entire burden of deterring or defeating threats to the homeland. Joining the ICC would bring precisely zero extra protection from genocide. Indeed, such a commitment may limit the US's freedom of action, discredit US deterrence, and actually make a devastating war more likely.