The nuclear family is a really new, weird, and unusual occurance. 

Throughout history, most families were intergenerational. The nuclear family only became a sort of norm in the post-war period with the growth of single-family housing and the suburbs. Jane Jacobs writes a lot about multi-generational families living in multi-family housing in cities in the mid 20th century. She notes that central planners for cities HATED these communities.

They thought they were messy.

They thought they were chaotic.

They thought they were crowded.

They were all of those things.

But they also provided a social support system that the nuclear family does not have.

They provided cheap and affordable childcare for young children while parents worked -- grandparents could easily look after children and provide food for the working parents. They gave children a place to see how the old should take a place in society and the old a place to see how the young are developing. They provided "eyes on the street," via grannies sitting on their stoops in the middle of the day, to keep neighborhoods safe and make it easier for children to play outside. They took a lot of the stress of raising a family off of raising a family.

They were the norm for quite some time. Even in my own family, my great-grandmother lived down the street from my grandmother and moved in with her when she was incredibly ill at the end of her life.

Nowadays, many adult children, brainwashed by the concept of the nuclear family, would simply lock their parents away in a nursing home and send their children to school to be nannied by the state.

There are lots of theories on what caused the breakdown of the extended family. I think urban planning was a huge part of it. Urban planners wanted to move people into clean and nice looking housing blocks or move them out to the atomistic suburbs. 

The Great Society also likely crowded out much of this apparent need for the extended family.

The sexual revolution and supposedly progressive reforms to divorce law broke apart families that might have otherwise stayed together for extended periods of time, gotten over their differences and disagreements, and decided to raise intergenerationally.

For myself - I'm striving to create an intergenerational household -- if not that, then an intergenerational neighborhood. There's little need for a family to be spread out across the entire country in pursuit of transient careers while grandma wastes away in a home because her children are too busy collecting gold stars.