People don’t lock their doors in many parts of America either like safe suburbs.
My house growing up couldn’t really even be properly locked up. You could lock the door but the first floor windows and sliding door on the deck didn’t lock at all.
If I notice the door is unlocked before bed I’ll lock it, but my neighborhood is very safe and I’m sure the locks often go unlocked before we go to sleep.
Before we moved here after our daughter was born, we lived in a rougher part of town and would instinctively lock the doors any time we went in our out. Our neighbor got robbed while she was at work, and my wife was at home when it happened, so we were very cautious. As a last resort we had a gun, which luckily we never had occasion to use.
It’s amazing how moving 10 miles away was such a huge stress reducer for me and my family.
It wasn't that long ago when there were only 4 unique keys in the world and a thief had a skeleton key that would replace them all. In those days nobody locked their door because there was no point. As lock technology got better people in places of crime installed good locks and told their friends to do the same. Most people have no need to lock their door, but nobody knows if they are the exception.
With all respect, I call bullshit to your "4 unique keys" theory. Locks have a long and varied history all around the world. There is no way a single skeleton key would even fit into an Arabic, Chinese, Indian, Venetian, Swiss, or German lock.
For practical purposes you are wrong. In any given city 4 door locks is about right. There are padlocks and other types of lock, but the subject here is door locks in one location so4 is close enough and was the truth in the US where my grandparents grew up.
My house growing up couldn’t really even be properly locked up. You could lock the door but the first floor windows and sliding door on the deck didn’t lock at all.