Why is equality between the sexes so rare in human societies?For anthropology
We cannot know this for sure because human behavior cannot be objectively understood. However, the most likely reason for this is because our societies have until very recently been shaped by natural forces and have been dependent on physical strength.
In past societies, we have had much less control over nature. We have not had machines to make our lives easier. We have not had birth control to allow women or couples to regulate when they have children. This forced societies to develop in ways that took these factors into account. Men and women came to have different roles because male strength was needed for many things and because women were often pregnant or nursing.
Out of these different roles arose ideologies about the respective natures of men and women. These ideologies are fading in today's technologically advanced world, but they have not faded yet because we have only been very technologically advanced for a relatively short time.