The thing with population density is that it's an average. Averages are notoriously useless for this sort of thing.

For example, the average density of the universe is 3-6 atoms per cubic meter. And that includes things like mountains, people, and stars.

When you're talking about provisioning broadband access, you have to provision for the worst case (or close enough that the differences are insigificant), not the average. Otherwise, you're looking at half the population being under- or unserved.