How can insurance companies include preexisting conditions without a health care mandate?

If there's no mandate, and insurance companies are forced to give me affordable health care regardless of preexisting conditions, that means I only have to get health insurance when I come down with cancer. I hear many Republicans stating that they are all for barring insurance companies from excluding those with preexisting conditions, but none of them explain how you can do so without a national mandate to buy health insurance like we have with auto insurance.