Why are black people so arrogant?

Overall, there are arrogant people in every race. What causes arrogance is a combination of observing it in others and being insecure. Arrogance helps to cover for insecurity.

As for why one ethnic group may have this trait more than others would usually involve socioeconomic factors. People who are extremely rich or extremely poor would be more likely than others to be arrogant. Being raised by a single-parent family in a low income neighborhood may cause insecurity, which tends to lead to arrogance or brash behavior as a way to distract from possible feelings of low self-worth.

So social influences and upbringing can contribute, and people within a particular ethnic group would be more likely to experience a lot of the same social pressures which help develop this trait. Anyone, regardless of race, would be more likely to develop the same character traits under the same conditions.