Why are so many African Americans in prison?

After our discussion in class of African Americans being oppressed in the legal system, and prisoners being used as cheap prison labor, I put some extended thought into this and discussed it with my boyfriend. We both had opposing views, but still overlapped in some points. I argued that African Americans are being singled out initially by "driving while black" by police officers, which my boyfriend did agree with me on, to prosecutions, and finally by unethically being used as cheap prison labor. Anyways, what my boyfriend was arguing is that because within city centers there are so many more people, businesses, and what not, that results in more activity, and ultimately more crime. The increase in crime leads to increase in law enforcement, which seems to make sense. We also know that within city centers the majority of people are made up by African Americans and non-whites. So maybe the police aren't necessarily singling out African Americas to pull over, but rather that just happens to be the majority of people within city centers. Maybe the cause of what we believe to be "driving while black" within cities, is really just do to socioeconomic status which contributes to where one lives. It's kind of confusing to get out what I'm trying to say if this makes any sense, but it's just an idea of what might contribute to the make-up of so many African Americans within prison. Although I do agree that racial stereotyping and discrimination plays a huge role in the outcome.