What you say is absolutely correct. But the real point of what would be considered public health insurance is that the risks are spread over a much larger group than simply a base of employees. The pricing mechanism is based on the risk of the people in the group. If the group is defined as the employees of the company, then that number is going to depend on the risk factors of the entire group.

If the group is the entire population of a country then you are going to pay a rate that is based on the risks of a very large population. If the people are on average healthy you will pay less. If the people on average are unhealthy you will pay more.