Why should the cost of insurance be optimized to the individual and not the aggregate? If the point of insurance is to flatten the price of health care to some

Why should the cost of insurance be optimized to the individual and not the aggregate? If the point of insurance is to flatten the price of health care to something everyone can afford, then the cost is the sum of all health care expenditures for a period, not the health and precrime health of Joe Mightgetrectalcancer.