When TOS was made, I don't think most people wore seat belts in cars. I don't know if most cars came with seat belts.
Click for Link...the first seat belt law was a federal law, Title 49 of the United States Code, Chapter 301, Motor Vehicle Safety Standard, which took effect on January 1, 1968 that required all vehicles (except buses) to be fitted with seat belts in all designated seating positions.