To put another way, the US has more public options (Medicare, Medicaid, VA) than those countries. (Or, say, Australia, which has a public option by default but which really, really, really encourages people to go private.)
Japan has a public option, although employees opt for it at the group rather than the individual level. A given company's employees choose their health insurance provider via their union/representatives, and they always have the option of the baseline national provider. (Non-employees/dependents always have public healthcare via their local government)