Well in some societies extended families live to together. I know of a few.
This is the case in our family. My grandmother always lived with us. Actually, my grandmother and mom (one widowed, the other divorced) lived together in a house they both owned and when my parents got married, my Dad moved in. So, I grew up always having a grandmother living with us, which was very nice for me.
Another example is my cousin. When her mother died, her Dad came to live with her and her husband and four boys. He lived with them until he passed away many years later. Also, I have ten nieces and nephews--my sister's kids. Several of them live together. One moved to California and one by one the other two followed. They have all lived together for many years now and are in their fifties.
My husband and I have two grown daughters and are in the process of building a one bedroom house behind our little three bedroom house. We plan to move into the one bedroom and our daughters are going to share the three bedroom. It's a preference for all of us, not a need. Everyone is financially stable.
It's not at all uncommon in Hawaii. Many Asians and polynesians have extended family living with, or right by them. Our neighbor is building a small house on his lot for his daughter, another neighbor lives in the same house with her grown children and grandchildren, and another neighbor lives with his sister, his wife and baby. On our street, it's as common as not.
Many of my husband's friends live with family--usually in a separate house on the same property. For instance, one friend and his brother built their house next to their parent's home on their property. The sons are able to help their elderly parents and they all benefit since many expenses such as property tax are shared. No one is mooching off of anyone else. Everyone contributes and they all enjoy each other's company.
For some reason, most WASPs view these living arrangements in a negative way. Someone is either thought of as not growing up, or mooching, or driving someone crazy. It's like you turn eighteen and you're out and if you return it's viewed as a personal failure and a nuisance. I don't really understand this way of thinking. It's not that way in many other cultures. I wonder why it's so different and how it came to be this way.