Maybe you didn't notice, but primitive cultures have a serious lack of intensive care units, incubators and ventilators. And with a serious jump mortality of both mother and child before full maturity, that's a recipe for genetic failure.
Note: What happens if a heifer is bred too early?
1. Increased costs associated with breeding heifers at younger ages. 2. Increased calf losses due to dystocia (calving difficulty) and related problems including the necessary management inputs for dealing with dystocia.
Primative societies learned not to breed their animals too early, why would they not carry that lesson to themselves?