This is a great question. You would think that we would be enlightened enough to treat all people with respect and equality. However, as you know this is not the case. I would suggest that there are two reasons why.
First, not all people are enlightened. There are some places in the world where people actually believe that women are inferior biologically, intellectually, and socially. For these people, education would be very importat, especially in the area of gender equality.
Second, there is also a percieved social threat when women get too powerful. Some men get very insecure. For this reason, they often put down women. This is a horrible trend, but it still continues even in the United States.
The concept of gender inequality bears a striking resemblance to the concept of racial inequality. Both have been a part of human existence for countless years. People have been taught for generations of the "rightness" of these ideas. Both have been justified on religious grounds such that people consider them the only "right" way. When concepts such as these become ingrained in our sense of values, they change very slowly, if at all.
The idea of women being the "weaker" sex has been around since hunter-gatherers when men were considered the "protectors." Biblically, women were considered to be not only weak, but a source of temptation; this idea dating from the temptation of Adam by Eve, after she herself was tempted. Adam was quick to blame his wife:
And the man said, The woman whom thou gavest to be with me, she gave me of the tree, and I did eat.
Indeed, the unfortunate Eve's name was morphed into "evil." Later, St. Paul in Ephesians admonished women to submit to their husbands:
Wives, submit yourselves unto your own husbands, as unto the Lord.
In more modern times, a woman's role as supportive but submissive became the norm as part of the "Cult of Domesticity." Catherine Beecher, wife of Lyman Beecher and mother of Harriet Beecher Stowe glorified a woman's role as keeper of hearth and home:
No statesman…had more frequent calls for wisdom, frankness, tact, discrimination, prudence and versatility of talent.
As late as World War II, women who had worked in factories and industry during the war years were told that it was their wifely (and patriotic) duty to surrender their jobs for the men returning from the war and return to the kitchen. In the 1950's the ideal woman was one who, according to Life magazine was
A 32 year old mother of four, who had married at sixteen, an excellent mother, volunteer, and home manager who made her own clothes, hosted dinner parties, sang in the church choir, worked with the school PTA and Campfire Girls, and was devoted to her husband. She attends club or charity meetings, drives the children to school, does the weekly grocery shopping, makes ceramics, and is planning to study French.
The concept is not entirely Western. The Quran also speaks of gender inequality in Islamic culture:
Men are the maintainers of women because Allah has made some of them to excel others and because they spend out of their property; the good women are therefore obedient.
Please bear in mind that in all these instances, this was considered not only normal but "right." Ideas of rightness and wrongness change exceptionally slowly, be it racial or gender equality.
The idea of gender equality is a modern concept which has taken hold, and is gaining ground. Even so, one cannot expect societal ideas with thousands of years of history behind them to change in a few short years. So, the reason for gender equality in this modern day and age is: we are working on it; we just haven't gotten there yet."