The problem is modern homes are for the most part hard to divide. Homes in the 30s often had very defined rooms connected with hallways. Modern homes include mostly open spaces with a few discrete rooms.

Open spaces are pretty easy to divide as long as they're horizontal. Vertical 'lawyer foyers' are a bit harder to use well.

A big reason why is because of the depression. We constructed very few houses in the 1930s and 1940s. What we started building in the 1950s is a completely different thing and why homes pre-war that are well maintained are so prized.

Most of the pre-war houses are gone, because they were crappy. The houses that are still around and prized, or have been divided into multiple units, were the houses that rich people lived in. The houses everyone else lived in were torn down long ago.

Of course there's selection bias. And what was considered a "home" of that era was very wide ranging. A lot of people lived in what was little better than a shack. and yes, many of the pre-war homes that are still standing were for rich people back then and are today. But there are a lot of pre-war homes in areas that are considered undesirable or even slums in many midwestern and east coast cities. Many of them modified for multi-tenancy, yes but also many still standing in a state of dilapidation as old homes cost money to maintain and modernize.

Anyways, most people didn't live in single family homes before the 1950's. Tenements, apartments, and packing multiple families into a single space was very common across classes of people that weren't rich. Single family homes and land ownership that is attainable is a vert post-war idea. The Communists built huge towers with shared spaces (which we also did for our urban lower classes) and we built sprawling subdivisions of vernacular single family homes with private spaces.

Also, from 1930 to about 1947 we built almost nothing due to the depression and then the war. Most of the tenements and apartments and shard living spaces were becoming old and broken down and this combined with the better living conditions a suburban home and a car could provide fueled the great migration out of the cities. Of course, the lower classes were left with the last-gen homes and unable to repair the through thrift or wealth.