What time period did Americans seldom locked their doors?
In small town Iowa, I went years without locking my doors. Left the keys in the vehicles too. Never had a problem.
In rural Ohio, in the 40s and 50s, and even into the 60s.
The 1960's, when I was a child. I grew up in a small city in the midwest. My parents finally started locking the doors after the cookie jar was emptied of treats several times by the neighbor kids. Nothing malicious ever happened, just missing cookies.
I still know of a few people in my area now that don't lock their doors. Of course, they're in a rural setting and know everyone around them.
I'd be uncomfortable doing that. The movie "The Texas Chainsaw Massacre" made quite an impression on me, lo those many years ago. :-)