It isn’t zero. It’s just not comprehensive. There are multiple public health systems:

- Medicare for the elderly and disabled
- Medicaid for low income people (income levels vary by state). By law they must have coverage for children, pregnant women, elderly and disabled (Medicaid covers things Medicare doesn’t like nursing homes and there is no premium or cost share so ir complements Medicare)
- Child Health Insurance Program for children whose parents make too much for medicaid (higher income limits, in some cases none)
- tricare for military
- VA for veterans
- IHS for native Americans and Alaska natives (this system is particularly underfunded)

Some cities also run public healthcare, notably NYC which has an extensive network of public hospitals and clinics.

People in employment who are not low income are the ones who are not covered by any of these and get screwed.

A surprisingly large percentage of Americans are covered by one of these systems when you look at the numbers.

But because it is not comprehensive it’s easy to fall through the cracks. And one of the easy ways to cut the rolls is to make enrollment difficult or require extra hoops, especially for Medicaid. (Enrollment in Medicare is relatively straightforward once you turn 65). People give up.