because its that type of car! i know a fair bit about cars, and i happen 2 know that the impala dosent have seatbelts, there are loads of cars that are the same!!!! i hope that helps in some way! (plus there bad नितंब, गधा 2:):)
I've often wondered that myself! They should be getting pulled over all the time for सीट बेल्ट violation! Maybe it's a particular mindset associated with the car. Hardly anyone used सीट belts back then, so maybe Kripke was going for that look of "freedom" associated with cars from that era. But they really should wear them!!
Yes, it's definitely possible that they are wearing them - they're quite easy to put on. The car definitely has lap belts though, the picture attached above is of the actual Hero car!