The group of people who hear the gospel en masse has a better chance of attaining to a higher proportion of individuals saved than the group which hasn't heard. These are logically distinct propositions. The more you know, the more you are responsible for. So I think that would even out the score quite a bit. The Christian portions of Africa are far more Christian now than America is.
People can always reject the gospel.
But that doesn't mean we therefore stop preaching it. This makes the Christian criteria of salvation unfair. This is not a difficulty, as explained. There are many factors. Your analysis is far too simplistic, but we have come to expect that from skeptical minds.