I would rather we don't have to spend precious emergency responder time on preventable injuries and deaths that occur at much higher rates by people not wearing seatbelts. If we can require you wear glasses to drive, we can require you wear a seatbelt as well. When someone dies in an accident because they didn't wear a seatbelt, there is another driver who now has to live with the fact that they killed someone.

If I'm in McDonald's and a fat person enters that does not increase my chances of gaining weight.

If I'm driving and a driver not wearing a seatbelt enters the road it does increase my chances of being in an accident because if something happens that makes them have to swerve sharply, such as a large animal leaping into the road in front of them, a seatbelt increases the chances they will be able to remain in control during that maneuver.

One of the primary aims of seatbelts are to stop you becoming a projectile and protect other occupants of your car and people outside it. It's not just a personal choice.