Why do you think that the gender gap in pay exists all over the world?
This is a great question. There are a few ways in which to answer this question. I think the best way to answer it is to look at it from the perspective of sociology. Sociologist like Peter Berger have made the point that people create society. More specifically, people in power create society. In this sense, all culture is constructed. It just so happens that the people who had most of the power were men. So, when men created society, they favored their gender at the expense of women. To be sure things are changing, but there are still places where the inequalities are enormous, like in the Middle East.
With that said, we can see this inequality in the United States as well. Recently, women brought a lawsuit against Walmart concerning this very issue.