From the time of America's founding, most American families were structurally the same. A man and woman would get married and have kids. When those kids grew up, they would not move too far away from home, would get married themselves and have kids of their own.
While this structure was not the only type of family, this so-called “nuclear family” could be presumed to be the case for most people. As a result, many of our laws, customs and institutions were based on the presumption that those were the types of families in which everyone lived.
This is even true regarding how we came to view care for the elderly, as the Wills, Trusts & Estates Prof Blog discussed in "Recent Study Finds Complex Family Structures Lead to More Complex Elder Care."
It was assumed that elderly people would have nearby family members, usually adult children, who could and would care for them. However, a recent study has found that with family structures getting more complex today, the presumption that an adult child will be around to care for an elderly person is no longer accurate.
This creates severe problems for many elderly people, since it is difficult for them to find and receive proper care.
As the population continues to age and families get more and more complex, this problem will only grow unless something is done to address it.
Reference: Wills, Trusts & Estates Prof Blog (April 17, 2017) "Recent Study Finds Complex Family Structures Lead to More Complex Elder Care."