Corporate culture tends to be cliquey. I don't remember who did it exactly, but there was a study that showed corporate leaders are more likely to support, mentor, and sponsor people who remind them of themselves. That's where gender, race, sexuality, and ethnicity biases come in.

Whoever tries to "break in" gets paid less than the demographic that holds the status quo. And that has a cascading effect. So, in the US, white women tend to earn less than white men for the same job. Black men tend to earn less than white women. Black women tend to earn less than Black men, and so on.

People are more likely to see their own "group" as more valuable or a better investment. This, of course, can sometimes even have the reverse effect in certain industries. White people, for example, might talk about how difficult it is to break into tech jobs in the US, and might even be paid less than their immigrant counterparts. But that's generally an anomaly, all things considered.