Where does this stem from, and I don't mean that in a negative way, its just that half a century ago salesman knocking at the door was pretty much routine. It's how people got their milk, newspaper, vacuums, carpets, shoes, plastic containers, make up, clothing, and so much more. Back then salesman were invited in and offered iced tea or coffee. So what's happened in the last 50rs or so.
ETA: Sorry if we've veered this interesting topic TV started off its course.
Perhaps....pre-Internet...? Conspiracy Theories began by simple face-to-face communication.
After all, this is how "religion" developed....