Americans and their insurance companies are insanely litigious. It is easy for most US-nationals because as individuals we have full-coverage insurance on our personal automobiles that extends primary coverage to rental cars. Your personal auto insurance in the UK, Australia, Japan, or wherever you are from likely wants nothing to do with a car accident in America.
Some international personal and corporate insurance policies extend primary coverage to rental cars in the US. If you have one of these policies, you will know it.