Because children watched them on 13"-25" tube TV's that were designed to make those imperfections look acceptable (for the time), that a modern display blows up to proportions never seen during production even with the best studio displays of the era.

semi-related: I'm visiting my parents with a Sony OLED, and the frame interpolation made parts of Monty Python and the Holy Grail look like it was shot on an HD video camera.