Woohoo!
This makes the assumption that people should automatically accept the Gospel when they hear it. Is this a reasonable assumption? It also assumes that merely hearing the Gospel gives a person a good enough chance of accepting salvation.
Imagine that it turns out that Islam was the correct religion. You have heard of Islam, right? You have heard some of their basic tenets. Do you think that you had just as much a chance as someone growing up in Saudi Arabia to have come to the conclusion that Islam was the correct religion? Do you think that if a Muslim person talked with you about becoming a Muslim, you would easily be able to convert? Would you feel that, simply because you know about Islam, that you have been given a sufficiently fair chance at accepting it?
I suspect not. So why do you expect someone who belongs to a "distant tribe" to accept Christianity, simply because a missionary might have told them the Gospel?
I'm not sure what you mean when you say that this is "down to humans, not God". Well, yes: it is about humans, and that's the point. It's pretty much human nature to accept the beliefs in which we are raised, and which permeate our culture. Those are the beliefs that we are generally going to find most comfortable or credible.
God knows that. So why would he come up with a plan of salvation that required people to accept it, when he knew that people wouldn't have an equal opportunity of accepting it?
Or, to include your objection: Why would God create a plan of salvation that depended upon other humans to ensure the salvation of other humans? I mean, it's not really fair that Ahmad didn't get the same chance at salvation as Christopher simply because Ahmad's parents were Muslim, and Christopher's were Christian.