If seatbelts are required everywhere in the US, or maybe in the world (no idea) why aren't cars built with an ignition interlock to the seatbelt?

Sure, it would be easy to overcome, but, most people would just put their seatbelts on before starting the car and driving, and isn't that what we want?

Maybe there is some ramifications to your car turning off if you unbuckle your belt, in a parking lot, or to reverse so you can twist your body.

Oh well, just thinking out loud