answer:Seat belts weren’t common in the US until the mid-1960s, and they didn’t become required equipment until the late 60’s (thanks LBJ). The original series started in 1966 – again, before the days when seat belts were required, and before they had widespread use. My guess is that the writers and producers didn’t even think about seat belts because the concept was so new, and it hadn’t become a ‘normal’ thing in American society. It didn’t even occur to them. @LuckyGuy, I think you’re analyzing based on 2016 norms – but you have to take into account the contemporary mores.