And these were real homes inhabited by people who had all this filth??
A lot of the calls we answered were triggered by complaints from neighbors and landlords.
German roaches aren't a native species, they aren't wild, don't just come inside like native roaches. They spread when bedding, luggage and furniture is moved, and if there is food, almost anything, they flourish. Another way I've seen them spread is when people carried stuff home in cardboard boxes, the roaches were under the overlapped flaps. A thin film of food crud on a floor will keep them fed.
Back then, early 70s, they had developed resistance to most common pesticides, we used Diazonon, and even then there were signs of resistant colonies. I don't know what is used today.