Some states have no seatbelt laws, or the seatbelt laws can not really be enforced. For example, in AZ you can not be pulled over for not wearing your seatbelt, they can only ticket you for it if they pulled you over for something else first.
We also have no law requiring helmet use on motorcycles at all here.
I still wear my seatbelt religiously, and insist on passengers doing the same.
Also, my car will beep if someone is in a seat and not buckled up if I am going over 15 MPH. Look at that, a free market response, without government intervention.
And no, I don't think any level of government has any business telling you to wear a seatbelt, regardless of age. Not that you need a "good" reason, but there are valid reasons to not wear a seatbelt while driving, not the least of which is to not put your seatbelt on right when you get in your car, and removing it about 1-2 minutes out from your destination, because those are the most likely times you will be assaulted in your vehicle, and seatbelts obviously interfere with a pistol draw. Of course you should practice removing your seatbelt and drawing your gun anyway, but that half second difference may be THE difference.