Why is gender the base of social inequality?

Back in the times of primitive man, humans often lived in groups or in pairs. Since females can reproduce, they become valuable. Females also have to raise their children. It was the male's duty to protect the female and his children and gather food. This is often seen in the animal world, especially with other primates. It is instinct.

As humans evolved, that fact stayed the same, as life was still very dangerous. Females are always valuable to a population. However, in modern times, life is not always dangerous, so that instinct/way of life is not applicable. But it still remains embedded in our pysche and dormant instincts.