Drilling only works if you have access to a garden where to drill. Any kind of apartment has to use the outdoor unit

If you're an individual with an apartment you don't have the choice to drill.

If you're building the apartment building you have the choice to drill for the entire building, and the number of units that benefit mean this is much more cost efficient than with single family homes.

In sensible countries each housing unit have a central heating solution, regardless of where the heat comes from.