Two main reasons. The first is that if you require everyone to have health insurance then they contribute to the entire polulation of possible insureds, which the larger the population of people who pay into the program the less everyone pays. Actuary’s calculate the premium required by a formula of previous actual claims of the covered population, investment returns on reserves and other income and operating expenses.
The second is that by paying into the insurance program if you become sick, the government will not have to reimburse hospitals for the money that you cannot pay, as hospitals seek payment or a reduction in taxes when it provides health care services for those who have no insurance or can’t pay.
This may be a little off but it’s close to reality which is why we cannot get rid of ACA commonly mis labled Obama Care.
We still can significantly reduce health care cost by addressing the pharmacutical Companies who sell drugs for half the price in other countries such as Canada..