> At the extreme, these things can hold a lot of people without too much land:

They can hold a lot of people without too much land, but they don't reduce the land requirements for transporting goods and supplies to and waste products away from those people.

Sure, you can hold a lot more people in San Francisco if you fill the city with high-rises, but that's not going to, e.g., suddenly give the city a greater supply of water to support that population, or a greater ability to handle the solid waste produced by that population.