I was referring to using the geothermal heat that's naturally just under the ground as free, passive heat like they do in Iceland. The geothermal we use in the US isn't the same, it's just digging down far enough where the earth is a relatively constant temperature, 50-60° F, which is used to both cool and heat a house or building by running a loop full of liquid to transfer heat or cool out into the loop field via what is basically an AC/heat pump system.
Another reason would be that you need a lot of land, at least an acre, for one house. It can't be done for apartments, in downtown areas, or neighborhoods where you can stand between two houses and reach out to touch them both.