Because its effects are not immediate, but rather occur over a long period of time. If you smoke, your risk is even greater. The only way to know if radon gas is present in the home is to test. Ask your home inspector to test for radon as a part of the home inspection process. If you are purchasing a home serviced by a private well, your lender may require that the well pass a water quality test prior to loan approval.
Some lenders only require a bacteria test.
Others require more extensive testing. Keep in mind that these tests are intended to ensure that the lender is not making a loan on a property with a faulty system, in case they have to repossess the property, not necessarily to protect the health of the residents. Hometest has been involved in several cases where a bacteria-only water test was performed to satisfy the lender and later, more testing showed that extensive testing should have been done. One recent case concerned a family whose lender only required a bacteria test. Since the house was, built in 1993, they didn't think there would be a problem, so they didn't ask for a complete water test.