One way to get a linear relationship between genetic and geographic distance is if the whole global human population was in a stable equilibrium with new mutations, migration, and genetic drift all balancing out. Why do the authors not think that this is a good explanation for humans?

There hasn't been enough time in human history to reach equilibrium

The genetic distance between humans is also correlated with heterozygosity, which wouldn't happen at equilibrium

The relationship between genetic and geographic distance only holds with the addition of 'waypoints' that humans had to migrate through, which wouldn't happen if humans were in equilibrium

The relationship between genetic and geographic distance changes when some populations indigenous to the Americas are removed, which wouldn't happen if these populations were in equilibrium