Because we have an expectation for more space. Pretty simple, no one wants to go backwards to when we lived in single room tenements with a wife pumping out babies for a decade in a row. Also, no one can afford a four bedroom home with twelve occupants on a single income like in the 50s.
You couldn't in the 50s either. You just made do.
That’s the real key to all this - if you want it, you make do, and learn to grab what you can.
Much worse than housing is the vehicle when you cross the 5 or 6 kid boundary, your available vehicles begin to rapidly plummet.