Commercial health insurance dominates the healthcare system in the US. In fact, the private sector operates the majority of US medical facilities as well. Public healthcare is only available for citizens and permanent residents over a certain age and, even then, it is limited in coverage.
In addition to that, healthcare is expensive. Because of its costliness, employer-provided health insurance is common among many US citizens as well as expats. However, being insured does not mean that your doctor appointments are going to be free. In most cases, you still have to cover a percentage of hospital fees out of pocket.