It's not just Africans. It doesn't make sense that Indian/Hispanic/Russian/American accents exist either. Earth ceased to exist hundreds of years ago, and there are no homogenous cultures anymore (except the va'ruun, their accent makes sense). In Bethesdas effort to be inclusive they treated accents like they're a racial attribute rather than cultural. In reality there would be a UC accent, a freestar accent, etc but the accents depicted in game would be long extinct.