There's no "logic" here, you're just not aware of the history of the term and the sociological history behind it.

The nuclear family was an oddity that developed in England concomitant to the Industrial Revolution in middle-class families for whom occupational relocation was common. It was enshrined as an ideal sociological familial arrangement in the United States because its normalization was conducive for developing larger pools of productive labor.

> It was enshrined as an ideal sociological familial arrangement in the United States because its normalization was conducive for developing larger pools of productive labor.

As opposed to pseudo-Confucius China where larger pools of productive labor naturally formed?

That doesn't take away anything from the fundamental point where it's the smallest self-replicating unit, logic on behalf of the participants has nothing to do with it because it works out the gate. Of course it isn't the best, it was developed during a time of struggle and turmoil a la the industrial revolution (for the rural poor), it won because it was the the most resilient model (small, mobile, reactive, etc) to hard times.

Edit: I said developed, if formed is a word that helps you understand that it's not conscious then here you go