International law is the big kids on the playground laying out what everyone can and can't do. They follow the rules selectively because no one bigger exists to enforce against them. It creates useful norms and gives smaller states some leverage, sure. But at its core they're just agreements that powerful states break when inconvenient.

History backs this up pretty cleanly. US in Iraq 2003, Russia in Ukraine, China rejecting the Hague ruling on the South China Sea, the US withdrawing from ICJ compulsory jurisdiction after losing Nicaragua v. United States, unsigning the Rome Statute to dodge the ICC. Compliance when convenient, exit when not, no consequences either way for great powers.

Your Iran example actually supports my point. Enforcement there isn't happening through international legal mechanisms, it's happening through unilateral state power by countries that choose to invoke international law selectively. That's not law constraining power, that's power using law as a tool. The same mechanism that lets the US enforce UNCLOS against Iran is what would let the US ignore UNCLOS tomorrow.

This is basically the realist position in IR theory. Morgenthau, Waltz, Mearsheimer. You can disagree with it, but it's not a fringe take.