Even if all aspects of two groups were equal -- sociability and preference to hire from one's own social circles -- you can still have a rich-get-richer effect.

Even if all aspects of two groups were equal -- sociability and preference to hire from one's own social circles -- you can still have a rich-get-richer effect. Group A has many more people in hiring authority positions for historical reasons, so continues to preferentially hire from their social group, which is also mostly group A. Meanwhile Group B is doing the exact same thing, but has fewer people in power so things don't equalize out.