Males shoved themselves into the mother-child relationship to create the nuclear "family" unit. Why?

They are parasites. He leeches off the effort the woman gives to her child, inserting himself like a brood parasite who demands she give him food and do his domestic work (and of course demands she give him sex.)

There was an amazing comment I read here months ago that clearly spelled it out, but I haven't been able to find it again.

Fathers attempt to supplant the baby in order to redirect benefits towards himself. Why do you think it's been normalized for babies to sleep in a crib away from the mother? For years science was targeted at proving why co-sleeping was bad, and only now is research being done that suggests that co-sleeping actually reduces the incidence of SIDS. But why was crib sleeping even popular to begin with?

Males will stop at nothing to ensure sexual access, and a mother sleeping with her baby disrupts that access. 

Fatherhood is actually detrimental to the child when you take everything into account. A father only serves to steal time, attention, and energy away from the child.

The only thing males have to offer as fathers is money, and the only reason money matters is because males built the system such that women would need to depend on them for it.

There is no animal in the world where the females act as indentured servants bringing the male sperm dispenser food and expending energy on him at the expense of her progeny.