It's also that it was an entirely different way of thinking. The extended family all lived under the same roof, all the aunts and uncles and grandparents and kids and counsins, and often all in the same room together. It allowed for social bonding, emotional intimacy, mutual education, and shared burden of child-rearing. To people used to sleeping side by side in straw pallets, animals close enough to share warmth would be completely natural.
A lot of people don't realize that the we've evolved for the extended family, and that the idea of the nuclear family is barely 150 years old -- and why modern culture is creating epidemics of social isolation, mental illness, ennui, and sociopathy.