These are people who appreciate the value of seat belts, but not every time they drive. They don't wear them for the short ride to the drug store, or in good weather, or when it might mess up their clothes, or when they are on familiar roads. They don't buckle up because they don't always perceive the risk as high enough to warrant use of a seat belt. It's illegal to drive without a seat belt in 49 states and the District of Columbia. Wear your seat belt or risk a ticket and fine.
Traffic crashes can happen at any moment.
The vast majority occur close to home, in your neighborhood, on your typical route to work, driving your kids to activities. You may be an excellent driver, but beware. There are other drivers on the road who are downright dangerous. Buckle up for those who would be left alone if you were to die. Your children, your wife or husband, your parents.