Several states have a requirement that health insurers issue policies to all people who apply for them, regardless of their health condition or whether they have a preexisting condition. This practice is known as guaranteed issue. Without appropriate safeguards in a state with guaranteed issue, individuals are able to wait until they get sick and then purchase coverage. Often, these individuals will drop coverage when the immediate need for health insurance has passed. This defeats the concept of insurance.
Health insurance is a means of protection from uncertain, future loss.
This mechanism relies upon a large number of similar participants sharing the risk of adverse consequences of unforeseeable events. Allowing people to purchase health insurance coverage for those who already are in need of medical services would eliminate this fundamental characteristic of insurance. Instead, people would be paying for predictable, definite conditions of which there is no uncertainty. In such a situation, there are not enough people who will not have a need for medical services subsidizing the costs of those who do need care. The end result is that health insurance rates will drastically rise as only those who know they will need medical services purchase coverage.