The idea of a single-family-home is a relatively new idea, cooked up in more recent times, during and post-industrial revolution. Ironically, everyone considers the nuclear family to be 'traditional' today but that is far from the truth.
The majority of the world and the majority of history shows that human homes tend to be extended, multigenerational dwellings or some other form of cohabitation. Running a household with only 2 adults and however many children is going to be stressful. It can take 2 adults just to run a home without children!
From a strictly mathematical standpoint, it makes sense to have more than two adults in a home. More labor hours per day means more income and more homemaking and more childrearing.