There have been many articles written about this, you should read them. But in some of the more egregious cases, I would ask you this: how do you sue a government for plundered art when the government has passed laws that prevent you from suing the government?
You've pointed to 'many' countries and also to laws stopping this. Now you've evaluated likelihood. Since you were able to drive a likelihood calculus, what specific laws were you calculating against?