Well-traveled places are already in the database, for the most part, so a lot of the benefit of signals like these is in filling in less-dense parts of the map. But yes, the amount of information we have about a particular place correlates strongly with population density.

Conveniently, many of the people in developing nations skipped the whole land-line thing and went straight to cell phones, albeit perhaps one per village, so we actually have data for places like sub-Saharan Africa.

Sending StreetView cars to sub-Saharan Africa is indeed expensive (although we've got StreetView kayaks, and StreetView backpacks, and there's now annotated StreetView of the International Space Station. Not sure about StreetView camels.) And we can work with satellite imagery when StreetView is too expensive.