I noted many 4+ adult families before WWII (from doing genealogy). It was a time when affordable housing for large families was generally obtainable and work/shopping/etc was in walking distance or reachable via public transit.

FF to today and affordable housing isn't available for families of any size. Most folks must shoulder a number of new major, ongoing expenses like individual transportation. Living is massively more complex due to reams of requirements attached to anything you can think of. Parenting time has increased from a few hours/week to 24/7 adulting and modern children fully their lives in a series of adult-curated, adult-populated boxes.

So we're back to needing 4+ typical income earners to meet minimal bills but nearly everything that enabled the 4+ adult household has been eradicated.