Would that be because the US wouldn’t want to be vulnerable to suits from non-US citizens anywhere in the world?
It’s because such suits have implications in foreign policy, which is the purview of the Executive...
For example, If the U.S. freezes Iranian assets (public and private) to try and coerce them into not developing nuclear weapons, isn’t that policy undermined when those assets are raided by the courts to compensate people who sue Iran in U.S. Court?
If we allow that, what was the purpose of freezing the assets? Why not just confiscate them?