Part of this can be explained by training technique and machine innovation
The bench press alone was not really a thing until the 1940/50s and not mainstream until the 70s. Arm isolation popularity same thing
Thus you saw a lot of guys with lean physiques and no chest/arms
These people were not skinny they were lean and athletic with good muscle tone, bone structure, and posture. They don’t really exist anymore due to modern food and weightlifting trends so people now are either very muscular, very skinny, or fat.
This is normal and healthy.