Why do health insurance plans require a medical exam?

Risk assessment: Not being in the insurance industry, i would surmise that a medical exam is required for risk assessment, especially if the plan is a for-profit one.

Insurers are in the: Business of collecting premiums and paying as few claims as possible. Some ask a few questions, deferring an exam, many want not only historical data, an exam, but urine and blood work. If they do not see what they like, you wll not be offered a policy.