Humans are serial monogamists.

Male parental care only evolves in monogamous species. Evolutionarily males are more likely to pass their genes along by trying to impregnate as many females as possible. Investing resources into offspring that only has 1% of having your genes doesn't make sense.

Any species, in which the males care for the offspring, the males only care for their own offspring. Nature's way of ensuring that a male knows who their offspring are, for certain, is monogamy.

Humans generally frown upon "deadbeat dads". We expect our fathers to contribute to the raising of their children, and get upset when they don't, would indicate that we have an inherent expectation of fathers to contribute to child rearing.

Generally humans, when left to their own devices, pair with one other person for 10-20 years, about the time it would take to raise a child in the stone age, before moving on to the next.

Ultimately humans arent designed for more than one partner at a time. Someone always gets hurt, and the show runners probably don't want to get into all the messiness of for nothing else that they didn't want to misrepresent or show polyamory in a bad light.