Because of the logarithmic scaling of infrastructure (like supermarkets, doctors, hospitals), and because of the square root scaling of people-density per area vs. total travel distance to said infrastructure.
>> moving goods instead of people scales logarithmically > This is obviously false. There's some average parcel size, and an associated maximum capacity for a delivery van.
You do move bananas by ship from continent to continent. Then you split it up, move it by smaller ships up-river. Then you split it up, move it by truck to each cities distribution center. Then you split it up, move it by smaller truck to each store. Then people buy it.
Obviously logarithmic and the whole reason for ships and freight trains to exist, otherwise we would all get our tropical fruit in person by airplane.
And since we are on a CS-heavy site, talking about scalability, constant factors are irrelevant. That is what scalability means. It is the extrapolation to big numbers, where those constants no longer matter anymore. Of course there might be a local equilibrium for sufficiently small numbers. But that is always temporary as humanity keeps growing.