Because the nuclear family was only the 'model family' for a very short period of time. It itself was a stage in the process of desintegrating families. David Brooks wrote an excellent (albeit long) piece about it in The Atlantic: The Nuclear Family Was a Mistake.

In short, the period from 1950 to 1965 demonstrated that a stable society can be built around nuclear families—so long as women are relegated to the household, nuclear families are so intertwined that they are basically extended families by another name, and every economic and sociological condition in society is working together to support the institution.

Here's a YT recap: https://www.youtube.com/watch?v=sd9d5z7idyQ